[XRay][compiler-rt] XRay Buffer Queue

This implements a simple buffer queue to manage a pre-allocated queue of
fixed-sized buffers to hold XRay records. We need this to support
Flight Data Recorder (FDR) mode. We also implement this as a sub-library
first to allow for development before actually using it in an
implementation.

Some important properties of the buffer queue:

- Thread-safe enqueueing/dequeueing of fixed-size buffers.
- Pre-allocation of buffers at construction.

This is a re-roll of the previous attempt to submit, because it caused
failures in arm and aarch64.

Reviewers: majnemer, echristo, rSerge

Subscribers: tberghammer, danalbert, srhines, modocache, mehdi_amini, mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D26232

llvm-svn: 288775