[M68k][compiler-rt] Allow compiler-rt builtins to be built for M68k (#169256)

I've tested this locally, and the builtins build proceeds without a
hitch for m68k-none-none. This is part of a larger effort to establish a
working m68k baremetal toolchain.

compiler-rt/cmake/builtin-config-ix.cmake

@@ -98,6 +98,7 @@ set(SPARCV9 sparcv9)
 set(WASM32 wasm32)
 set(WASM64 wasm64)
 set(VE ve)
+set(M68K m68k)
 
 if(APPLE)
   set(ARM64 arm64 arm64e)
@@ -109,7 +110,7 @@ set(ALL_BUILTIN_SUPPORTED_ARCH
   ${X86} ${X86_64} ${AMDGPU} ${ARM32} ${ARM64} ${AVR}
   ${HEXAGON} ${MIPS32} ${MIPS64} ${NVPTX} ${PPC32} ${PPC64}
   ${RISCV32} ${RISCV64} ${S390X} ${SPARC} ${SPARCV9}
-  ${WASM32} ${WASM64} ${VE} ${LOONGARCH64})
+  ${WASM32} ${WASM64} ${VE} ${LOONGARCH64} ${M68K})
 
 include(CompilerRTUtils)
 include(CompilerRTDarwinUtils)

compiler-rt/lib/builtins/CMakeLists.txt

@@ -881,6 +881,8 @@ set(ve_SOURCES
   ${GENERIC_TF_SOURCES}
   ${GENERIC_SOURCES})
 
+set(m68k_SOURCES ${GENERIC_SOURCES})
+
 add_custom_target(builtins)
 set_target_properties(builtins PROPERTIES FOLDER "Compiler-RT/Metatargets")