[XRay][compiler-rt] Use gettid instead of getpid in FDR mode.

Summary:
This was pointed out that FDR mode didn't quite put the thread ID in the
buffers, but instead would write down the parent process ID.

Reviewers: pelikan, rSerge

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D29484

llvm-svn: 294166