[XRay][compiler-rt] Runtime changes to support custom event logging

Summary:
This change implements support for the custom event logging sleds and
intrinsics at runtime. For now it only supports handling the sleds in
x86_64, with the implementations for other architectures stubbed out to
do nothing.

NOTE: Work in progress, uploaded for exposition/exploration purposes.

Depends on D27503, D30018, and D33032.

Reviewers: echristo, javed.absar, timshen

Subscribers: mehdi_amini, nemanjai, llvm-commits

Differential Revision: https://reviews.llvm.org/D30630

llvm-svn: 302857