[compiler-rt] Fix libFuzzer not building with pthreads on Windows (PR #109525)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

CC @devnexen 

This fix breaks with recent changes in libcxx, see https://github.com/llvm/llvm-project/pull/112094#issuecomment-2459614442.

I guess we could change this to simply not check things for libcxx - but that also puts us in an awkward position - then we know for certain that this code wouldn't simply build on top of libcxx using pthreads.

Alternatively we could skip the whole setting of thread names in mingw build configurations. As far as I understand, setting the thread name is just a nice-to-have around here. As we can't really confidently know the configuration (without relying on C++ standard library internals) it is safer to simply skip it. (I'm not aware of anybody using libfuzzer in such mingw configurations - Clang doesn't even support enabling the fuzzer sanitizer in mingw configurations anyway.)

https://github.com/llvm/llvm-project/pull/109525