[XRay] Merge instrumentation point table emission code into AsmPrinter.

Summary:
No need to have this per-architecture.  While there, unify 32-bit ARM's
behaviour with what changed elsewhere and start function names lowercase
as per the coding standards.  Individual entry emission code goes to the
entry's own class.

Fully tested on amd64, cross-builds on both ARMs and PowerPC.

Reviewers: dberris

Subscribers: aemerson, llvm-commits

Differential Revision: https://reviews.llvm.org/D28209

llvm-svn: 290858