[PowerPC] Restore stack ptr from frame ptr with setjmp

If a function happens to:

- call setjmp
- do a 16-byte stack allocation
- call a function that sets up a stack frame and longjmp's back

The stack pointer that is restores by setjmp will no longer point to a valid
back chain. According to the ABI, stack accesses in such a function are to be
frame pointer based - so it is an error (quite obviously) to restore the stack
from the back chain.
We already restore the stack from the frame pointer when there are calls to
fast_cc functions. We just need to also do that when there are calls to setjmp.
This patch simply does that.

This was pointed out by the Julia team.

Differential revision: https://reviews.llvm.org/D92906