[compiler-rt][XRay] re-submitting r276117, with fixes for build breakage due to extraneous and missing dependencies and attempts to build on unsupported OSes

Summary:
This is a fixed-up version of D21612, to address failure identified post-commit.

Original commit description:

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).

Fixes include:
- Gating XRay build to only Linux x86_64 and with the right dependencies in case it is the only library being built
- Including <cstddef> to fix std::size_t issue

Reviewers: kcc, rnk, echristo

Subscribers: mehdi_amini, llvm-commits

Differential Revision: https://reviews.llvm.org/D22611

llvm-svn: 276251

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