[XRay][compiler-rt] Use a single global volatile recursion guard for FDR handlers

Summary:
Before this change, the recursion guard for the flight data recorder
(FDR) mode handlers were independent. This change makes it so that when
a handler is already in the process of running and somehow the same or
another handler starts running -- say in a signal handler, while the
XRay handler is executing -- then we can use the same thread-local
recursion guard to stop the second handler from running.

Reviewers: kpw, eizan

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D37612

llvm-svn: 312992