Add support for X-Ray instrumentation

[jbush001]

http://llvm.org/docs/XRay.html

Need to add code to backend to emit tables. In NyuziAsmBackend, handle TargetOpcode::PATCHABLE_FUNCTION_ENTER, PATCHABLE_FUNCTION_EXIT, and PATCHABLE_TAIL_CALL.

Depends on Issue #88 (compiler-rt support)