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.