[clang] [compiler-rt] [llvm] [CMake][compiler-rt] Support for using compiler-rt atomic library (PR #106603)

[Petr Hosek via cfe-commits]

petrhosek wrote:

I'm building on Linux so I believe we use the atomic CAS spinlock. There's an option to use pthreads though:
https://github.com/llvm/llvm-project/blob/130eddf7a13f15c9c48b7fa7faf60e9bbee4f703/compiler-rt/lib/builtins/CMakeLists.txt#L240-L242
I'll check if enabling the use of pthreads would make this test pass, but it'd be better if we handled atomic CAS spinlocks as well since we would like to avoid the pthread dependency if possible.

https://github.com/llvm/llvm-project/pull/106603