[compiler-rt][XRay] Stash xmm registers in the trampolines

We now stash and restore the xmm registers in the trampolines so that
log handlers don't need to worry about clobbering these registers.

In response to comments in D21612.

Reviewers: rSerge, eugenis, echristo, rnk

Subscribers: mehdi_amini, llvm-commits

Differential Revision: https://reviews.llvm.org/D23051

llvm-svn: 277683