radv: Add ability to override the build id for the cache.

This would allow us to keep the shader cache key the same for updates
that we know won't impact compilation on SteamOS.

Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17426>

meson_options.txt

@@ -542,3 +542,9 @@ option(
   value : 'auto',
   description : 'build gallium d3d12 with video support.',
 )
+option(
+  'radv-build-id',
+  type : 'string',
+  value : '',
+  description : 'Override build id for shader cache keys (hex string). Can be extracted with readelf -x .note.gnu.build-id'
+)