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

[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