[PPC64, TSAN] Provide setjmp interceptor support for PPC64

This patch provides the assembly support for setjmp/longjmp for use
with the thread sanitizer.  This is a big more complicated than for
aarch64, because sibcalls are only legal under our ABIs if the TOC
pointer is unchanged.  Since the true setjmp function trashes the TOC
pointer, and we have to leave the stack in a correct state, we emulate
the setjmp function rather than branching to it.

We also need to materialize the TOC for cases where the _setjmp code
is called from libc.  This is done differently under the ELFv1 and
ELFv2 ABIs.

llvm-svn: 255059