[TSan] Improve handling of stack pointer mangling in {set,long}jmp, pt.8

Refine longjmp key management.  For Linux, re-implement key retrieval in
C (instead of assembly).  Removal of `InitializeGuardPtr` and a final
round of cleanups will be done in the next commit.

Reviewed By: dvyukov

Differential Revision: https://reviews.llvm.org/D64092

llvm-svn: 365513