radv: Synchronize shader compilation when dumping shaders
When RADV_DEBUG=shaders
is set, printing e.g. different NIR shaders from
different threads at the same time makes the output unreadable. Use a
global mutex to synchronize shader dumping so that all shaders get
printed in once piece.
Since we're writing everything to a file or terminal anyway, the performance impact of forcing singlethreaded compilation is negligible.