[XRay][compiler-rt] Stash flags as well in x86_64 trampoline

Summary:
This change saves and restores the full flags register in x86_64 mode.
This makes running instrumented signal handlers safer, and avoids flags
set during the execution of the event handlers from polluting the
instrumented call's flags state.

Reviewers: kpw, eizan, jfb

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D51277

llvm-svn: 340812