[XRay][compiler-rt] Always place the CPU record first for every buffer

Summary:
In FDR Mode, when we set up a new buffer for a thread that's just
overflowed, we must place the CPU identifier with the TSC record as the
first record. This is so that we can reconstruct all the function
entry/exit with deltas rooted on a TSC record for the CPU at the
beginning of the buffer.

Without doing this, the tools are rejecting the log for cases when we've
overflown and have different buffers that don't have the CPU and TSC
records as the first entry in the buffers.

Reviewers: pelikan

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D38995

llvm-svn: 315987