Revert "[compiler-rt] Compile assembly files as ASM not C"

This reverts commit d58fd4e5. This broke
compiler-rt compilation on macOS:

codesign --sign - /Users/buildslave/jenkins/workspace/lldb-cmake/lldb-build/lib/clang/12.0.99/lib/darwin/libclang_rt.tsan_ios_dynamic.dylib
ld: warning: ignoring file projects/compiler-rt/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_64
ld: warning: ignoring file projects/compiler-rt/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
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