[XRay][compiler-rt][NFC] Deduplicate code in x86-64 trampolines.

Summary:
The layout of all registers saved on stack shouldn't deviate and will be reused in future trampolines as well.

While there, fix whitespace and clarify comments.

Author: mpel

Reviewers: dberris

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D27799

llvm-svn: 289789