radv: Synchronize shader compilation when dumping shaders

Friedrich Vock requested to merge pixelcluster/mesa:radv-shader-dump-locked into main

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.

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