build: always add support for assembly targets on Darwin

[compnerd]

Summary

When attempting to build compiler-rt on a developer transition kit, the
build would fail due to .S files not being handled properly by the
Ninja generator. Rather than conditionalising on Xcode, conditionalise
to Darwin. Because we know that the system compiler is clang based, it
will always properly handle the pre-processing based on the extension.

[thegameg]

Hi @compnerd, it looks like this fails to link: http://green.lab.llvm.org/green/job/clang-stage1-RA/13171/console

Undefined symbols for architecture arm64:
  "_wrap__setjmp", referenced from:
      substitution__setjmp in tsan_interceptors_posix.cpp.o
  "_wrap_setjmp", referenced from:
      substitution_setjmp in tsan_interceptors_posix.cpp.o
  "_wrap_sigsetjmp", referenced from:
      substitution_sigsetjmp in tsan_interceptors_posix.cpp.o
ld: symbol(s) not found for architecture arm64

I'm thinking the warnings above are part of the issue:

ld: warning: ignoring file lib/tsan/CMakeFiles/clang_rt.tsan_ios_dynamic.dir/rtl/tsan_rtl_amd64.S.o, building for iOS-arm64 but attempting to link with file built for iOS Simulator-x86_64ld: warning: ignoring file lib/tsan/CMakeFiles/clang_rt.tsan_ios_dynamic.dir/rtl/tsan_rtl_aarch64.S.o, building for iOS-arm64 but attempting to link with file built for iOS Simulator-x86_64

compiler-rt/cmake/Modules/AddCompilerRT.cmake

if(COMMAND runtime_register_component)
     runtime_register_component(${name})
   endif()
   add_dependencies(compiler-rt ${name})
 endfunction()
 
 function(add_asm_sources output)
   set(${output} ${ARGN} PARENT_SCOPE)
   # Xcode will try to compile asm files as C ('clang -x c'), and that will fail.
-  if (${CMAKE_GENERATOR} STREQUAL "Xcode")
+  if(CMAKE_SYSTEM_NAME STREQUAL Darwin)
     enable_language(ASM)
   else()
     # Pass ASM file directly to the C++ compiler.
     set_source_files_properties(${ARGN} PROPERTIES LANGUAGE C)
   endif()
 endfunction()
 
 macro(set_output_name output name arch)