[TSan] Improve handling of stack pointer mangling in {set,long}jmp, pt.2

Switch `LongJmp` over to lookup JmpBuf via plain old (unmangled) SP.
This makes the computation of mangled SPs in the TSan assembly files
unnecessary, which will be cleaned up in follow-up revisions.

Reviewed By: dvyukov

Differential Revision: https://reviews.llvm.org/D63942

llvm-svn: 364818