[CMake][compiler-rt][libunwind] Compile assembly files as ASM not C, unify workarounds

It isn't very wise to pass an assembly file to the compiler and tell it to compile as a C file and hope that the compiler recognizes it as assembly instead.
Simply don't mark the file as C and CMake will recognize the rest.

This was attempted earlier in https://reviews.llvm.org/D85706, but reverted due to architecture issues on Apple.
Subsequent digging revealed a similar change was done earlier for libunwind in https://reviews.llvm.org/rGb780df052dd2b246a760d00e00f7de9ebdab9d09.
Afterwards workarounds were added for MinGW and Apple:
* https://reviews.llvm.org/rGb780df052dd2b246a760d00e00f7de9ebdab9d09
* https://reviews.llvm.org/rGd4ded05ba851304b26a437896bc3962ef56f62cb

The workarounds in libunwind and compiler-rt are unified and comments added pointing to each other.
The workaround is updated to only be used for MinGW for CMake versions before 3.17, which fixed the issue (https://gitlab.kitware.com/cmake/cmake/-/merge_requests/4287).

Additionally fixed Clang not being passed as the assembly compiler for compiler-rt runtime build.

Example error:
[525/634] Building C object lib/tsan/CMakeFiles/clang_rt.tsan-aarch64.dir/rtl/tsan_rtl_aarch64.S.o
FAILED: lib/tsan/CMakeFiles/clang_rt.tsan-aarch64.dir/rtl/tsan_rtl_aarch64.S.o
/opt/tooling/drive/host/bin/clang --target=aarch64-linux-gnu -I/opt/tooling/drive/llvm/compiler-rt/lib/tsan/.. -isystem /opt/tooling/drive/toolchain/opt/drive/toolchain/include -x c -Wall -Wno-unused-parameter -fno-lto -fPIC -fno-builtin -fno-exceptions -fomit-frame-pointer -funwind-tables -fno-stack-protector -fno-sanitize=safe-stack -fvisibility=hidden -fno-lto -O3 -gline-tables-only -Wno-gnu -Wno-variadic-macros -Wno-c99-extensions -Wno-non-virtual-dtor -fPIE -fno-rtti -Wframe-larger-than=530 -Wglobal-constructors --sysroot=. -MD -MT lib/tsan/CMakeFiles/clang_rt.tsan-aarch64.dir/rtl/tsan_rtl_aarch64.S.o -MF lib/tsan/CMakeFiles/clang_rt.tsan-aarch64.dir/rtl/tsan_rtl_aarch64.S.o.d -o lib/tsan/CMakeFiles/clang_rt.tsan-aarch64.dir/rtl/tsan_rtl_aarch64.S.o -c /opt/tooling/drive/llvm/compiler-rt/lib/tsan/rtl/tsan_rtl_aarch64.S
/opt/tooling/drive/llvm/compiler-rt/lib/tsan/rtl/tsan_rtl_aarch64.S:29:1: error: expected identifier or '('
.section .text
^
1 error generated.

Differential Revision: https://reviews.llvm.org/D86308

Author: tambry

clang/runtime/CMakeLists.txt

@@ -75,6 +75,7 @@ if(LLVM_BUILD_EXTERNAL_COMPILER_RT AND EXISTS ${COMPILER_RT_SRC_ROOT}/)
     CMAKE_ARGS ${CLANG_COMPILER_RT_CMAKE_ARGS}
                -DCMAKE_C_COMPILER=${LLVM_RUNTIME_OUTPUT_INTDIR}/clang
                -DCMAKE_CXX_COMPILER=${LLVM_RUNTIME_OUTPUT_INTDIR}/clang++
+               -DCMAKE_ASM_COMPILER=${LLVM_RUNTIME_OUTPUT_INTDIR}/clang
                -DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE}
                -DCMAKE_MAKE_PROGRAM=${CMAKE_MAKE_PROGRAM}
                -DLLVM_CONFIG_PATH=${LLVM_RUNTIME_OUTPUT_INTDIR}/llvm-config

compiler-rt/cmake/Modules/AddCompilerRT.cmake

@@ -109,11 +109,11 @@ 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")
-    enable_language(ASM)
-  else()
-    # Pass ASM file directly to the C++ compiler.
+  # CMake doesn't pass the correct architecture for Apple prior to CMake 3.19. https://gitlab.kitware.com/cmake/cmake/-/issues/20771
+  # MinGW didn't work correctly with assembly prior to CMake 3.17. https://gitlab.kitware.com/cmake/cmake/-/merge_requests/4287 and https://reviews.llvm.org/rGb780df052dd2b246a760d00e00f7de9ebdab9d09
+  # Workaround these two issues by compiling as C.
+  # Same workaround used in libunwind. Also update there if changed here.
+  if((APPLE AND CMAKE_VERSION VERSION_LESS 3.19) OR (MINGW AND CMAKE_VERSION VERSION_LESS 3.17))
     set_source_files_properties(${ARGN} PROPERTIES LANGUAGE C)
   endif()
 endfunction()

libunwind/src/CMakeLists.txt

@@ -24,14 +24,10 @@ set(LIBUNWIND_ASM_SOURCES
     UnwindRegistersRestore.S
     UnwindRegistersSave.S
     )
-if (MINGW OR APPLE)
-  # CMake doesn't build assembly sources for windows/gnu targets properly
-  # (up to current CMake, 3.16), so treat them as C files.
-  # Additionally, CMake ignores OSX_ARCHITECTURE for ASM files when targeting
-  # Apple platforms.
-  set_source_files_properties(${LIBUNWIND_ASM_SOURCES}
-                              PROPERTIES
-                                LANGUAGE C)
+
+# See add_asm_sources() in compiler-rt for explanation of this workaround.
+if((APPLE AND CMAKE_VERSION VERSION_LESS 3.19) OR (MINGW AND CMAKE_VERSION VERSION_LESS 3.17))
+  set_source_files_properties(${LIBUNWIND_ASM_SOURCES} PROPERTIES LANGUAGE C)
 endif()
 
 set(LIBUNWIND_HEADERS