[compiler-rt][XRay] Implement __xray_unpatch() and __xray_remove_handler()

Summary:
We also add one test (and the XRay testing infrastructure) to exercise
the patching and unpatching code. This uses the XRay API exported
through the headers as well, installing a custom log handler.

Depends on D23101 for the updated emitted code alignment for the
return/entry sleds.

Reviewers: rSerge, echristo, rnk

Subscribers: tberghammer, danalbert, srhines, mehdi_amini, llvm-commits

Differential Revision: https://reviews.llvm.org/D23154

llvm-svn: 277971