riscv64: add rv64gc bare-metal sample

This sample works well for building the open-source first stage
bootloader for the SiFive U540 device (and similar):

https://github.com/sifive/freedom-u540-c000-bootloader

Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>

samples/riscv64-unknown-elf/crosstool.config

@@ -0,0 +1,7 @@
+CT_EXPERIMENTAL=y
+CT_ARCH_RISCV=y
+# CT_DEMULTILIB is not set
+CT_ARCH_USE_MMU=y
+CT_ARCH_64=y
+CT_DEBUG_GDB=y
+# CT_GDB_CROSS_PYTHON is not set

samples/riscv64-unknown-elf/reported.by

@@ -0,0 +1,3 @@
+reporter_name="Paul Walmsley <paul.walmsley@sifive.com>"
+reporter_url="https://www.sifive.com/"
+reporter_comment=""