util: move pipe_barrier into src/util and rename to util_barrier

The #if guard is probably not 100% equivalent to the previous PIPE_OS
check, but if anything it should be an over-approximation (are there
pthread implementations without barriers?), so people will get either
a good implementation or compile errors that are easy to fix.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
5 files changed