[tsan] Adjust setjmp/longjmp handling on Darwin for macOS Mojave

On macOS Mojave, the OS started using the XOR-by-a-secret-key scheme (same as glibc is alread doing) for storing the SP value in setjmp environment. We need to adjust for that to keep supporting setjmp/longjmp on latest Darwin. The patch is basically doing the same what we're already doing for glibc.

rdar://problem/43542596

Differential Revision: https://reviews.llvm.org/D51064

llvm-svn: 340350