[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