llvm: Only use GCC include paths when cross compiling

native should only use the compiler defaults for system includes.

cpu/Makefile.include.llvm

@@ -24,11 +24,12 @@ export DBG         = $(GDBPREFIX)gdb
 # LLVM lacks a binutils strip tool as well...
 #export STRIP      = $(LLVMPREFIX)strip
 
-# Clang on Linux uses GCC's C++ headers and libstdc++ (installed with GCC)
-# Ubuntu and Debian use /etc/alternatives/gcc-$(TARGET_ARCH)-include/c++/$(GCC_VERSION)
-# Arch uses /usr/$(TARGET_ARCH)/include/c++/$(GCC_VERSION)
-# Gentoo uses /usr/lib/gcc/$(TARGET_ARCH)/$(GCC_VERSION)/include/g++-v5
-GCC_CXX_INCLUDE_PATTERNS ?= \
+ifneq (,$(TARGET_ARCH))
+  # Clang on Linux uses GCC's C++ headers and libstdc++ (installed with GCC)
+  # Ubuntu and Debian use /etc/alternatives/gcc-$(TARGET_ARCH)-include/c++/$(GCC_VERSION)
+  # Arch uses /usr/$(TARGET_ARCH)/include/c++/$(GCC_VERSION)
+  # Gentoo uses /usr/lib/gcc/$(TARGET_ARCH)/$(GCC_VERSION)/include/g++-v5
+  GCC_CXX_INCLUDE_PATTERNS ?= \
   /etc/alternatives/gcc-$(TARGET_ARCH)-include/c++/*/ \
   /usr/$(TARGET_ARCH)/include/c++/*/ \
   /usr/lib/gcc/$(TARGET_ARCH)/*/include/g++-v8 \
@@ -37,58 +38,57 @@ GCC_CXX_INCLUDE_PATTERNS ?= \
   /usr/lib/gcc/$(TARGET_ARCH)/*/include/g++-v5 \
   #
 
-# Try to find the proper multilib directory using GCC, this may fail if a cross-
-# GCC is not installed.
-ifeq ($(GCC_MULTI_DIR),)
-  GCC_MULTI_DIR := $(shell $(PREFIX)gcc $(CFLAGS) -print-multi-directory 2>/dev/null)
-endif
+  # Try to find the proper multilib directory using GCC, this may fail if a cross-
+  # GCC is not installed.
+  ifeq ($(GCC_MULTI_DIR),)
+    GCC_MULTI_DIR := $(shell $(PREFIX)gcc $(CFLAGS) -print-multi-directory 2>/dev/null)
+  endif
 
-ifneq (,$(TARGET_ARCH))
   # Tell clang to cross compile
   export CFLAGS     += -target $(TARGET_ARCH)
   export CXXFLAGS   += -target $(TARGET_ARCH)
   export LINKFLAGS  += -target $(TARGET_ARCH)
-endif
 
-# Use the wildcard Makefile function to search for existing directories matching
-# the patterns above. We use the -isystem gcc/clang argument to add the include
-# directories as system include directories, which means they will not be
-# searched until after all the project specific include directories (-I/path)
-# We sort the list of found directories and take the last one, it will likely be
-# the most recent GCC version. This avoids using old headers left over from
-# previous tool chain installations.
-GCC_CXX_INCLUDES ?= \
-    $(addprefix \
-        -isystem $(firstword \
-            $(foreach pat, $(GCC_CXX_INCLUDE_PATTERNS), $(lastword $(sort $(wildcard $(pat)))))), \
-        /. /$(TARGET_ARCH)/$(GCC_MULTI_DIR) /backward \
-    )
+  # Use the wildcard Makefile function to search for existing directories matching
+  # the patterns above. We use the -isystem gcc/clang argument to add the include
+  # directories as system include directories, which means they will not be
+  # searched until after all the project specific include directories (-I/path)
+  # We sort the list of found directories and take the last one, it will likely be
+  # the most recent GCC version. This avoids using old headers left over from
+  # previous tool chain installations.
+  GCC_CXX_INCLUDES ?= \
+      $(addprefix \
+          -isystem $(firstword \
+              $(foreach pat, $(GCC_CXX_INCLUDE_PATTERNS), $(lastword $(sort $(wildcard $(pat)))))), \
+          /. /$(TARGET_ARCH)/$(GCC_MULTI_DIR) /backward \
+      )
 
-# If nothing was found we will try to fall back to searching for a cross-gcc in
-# the current PATH and use a relative path for the includes
-ifeq (,$(GCC_CXX_INCLUDES))
-  GCC_CXX_INCLUDES := $(addprefix -isystem ,$(wildcard $(dir $(shell which $(PREFIX)gcc))../$(TARGET_TRIPLE)/include))
-endif
+  # If nothing was found we will try to fall back to searching for a cross-gcc in
+  # the current PATH and use a relative path for the includes
+  ifeq (,$(GCC_CXX_INCLUDES))
+    GCC_CXX_INCLUDES := $(addprefix -isystem ,$(wildcard $(dir $(shell which $(PREFIX)gcc))../$(TARGET_TRIPLE)/include))
+  endif
 
-# Pass the includes to the C++ compilation rule in Makefile.base
-export CXXINCLUDES += $(GCC_CXX_INCLUDES)
+  # Pass the includes to the C++ compilation rule in Makefile.base
+  export CXXINCLUDES += $(GCC_CXX_INCLUDES)
 
-# Some C headers (e.g. limits.h) are located with the GCC libraries
-GCC_C_INCLUDE_PATTERNS ?= \
-  /usr/lib/gcc/$(TARGET_TRIPLE)/*/ \
-  #
+  # Some C headers (e.g. limits.h) are located with the GCC libraries
+  GCC_C_INCLUDE_PATTERNS ?= \
+    /usr/lib/gcc/$(TARGET_TRIPLE)/*/ \
+    #
 
-GCC_C_INCLUDES ?= \
-    $(addprefix -isystem ,$(wildcard $(addprefix \
-        $(lastword $(sort \
-            $(foreach pat, $(GCC_C_INCLUDE_PATTERNS), $(wildcard $(pat))))), \
-        include include-fixed) \
-    ))
+  GCC_C_INCLUDES ?= \
+      $(addprefix -isystem ,$(wildcard $(addprefix \
+          $(lastword $(sort \
+              $(foreach pat, $(GCC_C_INCLUDE_PATTERNS), $(wildcard $(pat))))), \
+          include include-fixed) \
+      ))
 
-# If nothing was found we will try to fall back to searching for the libgcc used
-# by an installed cross-GCC and use its headers.
-ifeq (,$(GCC_C_INCLUDES))
-  GCC_C_INCLUDES := $(addprefix -isystem ,$(wildcard $(addprefix $(dir $(shell $(PREFIX)gcc -print-libgcc-file-name)), include include-fixed)))
-endif
+  # If nothing was found we will try to fall back to searching for the libgcc used
+  # by an installed cross-GCC and use its headers.
+  ifeq (,$(GCC_C_INCLUDES))
+    GCC_C_INCLUDES := $(addprefix -isystem ,$(wildcard $(addprefix $(dir $(shell $(PREFIX)gcc -print-libgcc-file-name)), include include-fixed)))
+  endif
 
-export INCLUDES += $(GCC_C_INCLUDES)
+  export INCLUDES += $(GCC_C_INCLUDES)
+endif