[compiler-rt] [XRay] Basic initialization and flag definition for XRay runtime

Summary:
This patch implements the initialisation and patching routines for the XRay runtime, along with the necessary trampolines for function entry/exit handling. For now we only define the basic hooks for allowing an implementation to define a handler that gets run on function entry/exit. We expose a minimal API for controlling the behaviour of the runtime (patching, cleanup, and setting the handler to invoke when instrumenting).

Depends on D19904

Reviewers: echristo, kcc, rnk

Subscribers: rnk, mehdi_amini, llvm-commits

Differential Revision: https://reviews.llvm.org/D21612

llvm-svn: 276117