Add support for AUDIO_UTILS_FIFO_SYNC_SINGLE_THREADED

This will be used for simple non-atomic (single-threaded) applications
of the FIFO library, and should offer slightly better performance due to
removal of unnecessary memory barriers.  Performance will be confirmed
by benchmark.

Test: not yet, will be used by libdataflow for single-threaded graphs
Change-Id: I71b5205691adf561108c0d564b63435a01c04b91
3 files changed