[XRay][compiler-rt] Refactor recursion guard for Basic and FDR Mode

Summary:
This change extracts the recursion guard implementation from FDR Mode
and updates it to do the following:

- Do the atomic operation correctly to be signal-handler safe.

- Make it usable in both FDR and Basic Modes.

Before this change, the recursion guard relied on an unsynchronised read
and write on a volatile thread-local. A signal handler could then run in
between the read and the write, and then be able to run instrumented
code as part of the signal handling. Using an atomic exchange instead
fixes that by doing a proper mutual exclusion even in the presence of
signal handling.

Reviewers: kpw, eizan, jfb

Reviewed By: eizan

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D47696

llvm-svn: 334064