Only select GDB version if GDB is enabled

... which fixes a bogus warning when configuration without GDB is
loaded.

Signed-off-by: Alexey Neyman <stilor@att.net>

config/arch/riscv.in

@@ -12,7 +12,7 @@
 ## select ARCH_SUPPORTS_WITH_TUNE
 ## select GCC_REQUIRE_7_or_later
 ## select BINUTILS_REQUIRE_2_28_or_later
-## select GDB_REQUIRE_8_0_or_later
+## select GDB_REQUIRE_8_0_or_later if DEBUG_GDB
 
 ## help The RISC-V architecture, as defined by:
 ## help     http://www.riscv.org/