[compiler-rt] [compiler-rt] Destroy pthread attrs after use in tests (PR #114923)

[Kyle Evans via llvm-commits]

kevans91 wrote:

> LGTM.
> 
> Specifically https://pubs.opengroup.org/onlinepubs/007904875/functions/pthread_create.html:
> 
> > If the attributes specified by attr are modified later, the thread's attributes shall not be affected

Right, the spec doesn't really allow for any possibility that isn't copying the `attr` passed in out of the harm's way.

https://github.com/llvm/llvm-project/pull/114923