[compiler-rt] [rtsan] Explicitly link pthread in the tests and dynamic libraries (PR #98256)

[Chris Apple via llvm-commits]

cjappl wrote:

FYI I do not have a powerpc build to test on, so this is just a hunch. I do think all the signs point to this being the fix, though.

https://github.com/llvm/llvm-project/pull/98256