[XRay][compiler-rt] Explicitly initialise members.

Before this, the change committed in D26232 might have an uninitialised
std::atomic<bool> that may or may not have a valid state. On aarch64
this breaks consistently, while it doesn't manifest as a problem in
x86_64.

This is an attempt to un-break this in aarch64.

llvm-svn: 288776