[XRay][compiler-rt] Fix up CFI annotations and stack alignment

Summary:
Previously, we had been very undisciplined about CFI annotations with
the XRay trampolines. This leads to runtime crashes due to mis-alined
stack pointers that some function implementations may run into (i.e.
those using instructions that require properly aligned addresses coming
from the stack). This patch attempts to clean that up, as well as more
accurately use the correct amounts of space on the stack for stashing
and un-stashing registers.

Reviewers: eugenis, kcc

Subscribers: kpw, llvm-commits

Differential Revision: https://reviews.llvm.org/D32202

llvm-svn: 300660