[compiler-rt] [tsan] Fix nested signal handling (PR #138599)

[Jakob Widauer via llvm-commits]

jwidauer wrote:

@dvyukov Thanks for your quick review!

I added a commit to store and restore the old signal set stack in the LongJmp and SetJmp methods. Is this similar to what you had in mind? What  do you think?

I also ran all the tsan tests multiple times and didn't get any failures.

https://github.com/llvm/llvm-project/pull/138599