[XRay][compiler-rt] Remove reliance on C++ ABI features


[XRay][compiler-rt] Remove reliance on C++ ABI features

This fixes http://llvm.org/PR32274.

This change adds a test to ensure that we're able to link XRay modes and
the runtime to binaries that don't need to depend on the C++ standard
library or a C++ ABI library. In particular, we ensure that this will work
with C programs compiled+linked with XRay.

To make the test pass, we need to change a few things in the XRay
runtime implementations to remove the reliance on C++ ABI features. In
particular, we change the thread-safe function-local-static
initialisation to use pthread_* instead of the C++ features that ensure
non-trivial thread-local/function-local-static initialisation.

Depends on D47696.

Reviewers: dblaikie, jfb, kpw, eizan

Reviewed By: kpw

Subscribers: echristo, eizan, kpw, mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D46998