[XRay][CodeGen] Use PIC-friendly code in XRay sleds and remove synthetic references in .text

Summary:
This is a re-roll of D36615 which uses PLT relocations in the back-end
to the call to __xray_CustomEvent() when building in -fPIC and
-fxray-instrument mode.

Reviewers: pcc, djasper, bkramer

Subscribers: sdardis, javed.absar, llvm-commits

Differential Revision: https://reviews.llvm.org/D37373

llvm-svn: 312466