[XRay] Refactor TSC related functions into a single header. NFC.

Summary:
The implementation, however, is in different arch-specific files, unless it's emulated.

Reviewers: dberris, pelikan, javed.absar

Subscribers: aemerson, llvm-commits

Differential Revision: https://reviews.llvm.org/D29796

llvm-svn: 294777