asahi: Precompile for shader-db

This gets shader-db's runner working, in conjunction with a shader-db ./run
modified to set ASAHI_MESA_DEBUG=precompile. This flag triggers precompiles of
all shaders witha default key so we can exercise the compiler.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18813>

src/asahi/lib/agx_device.h

@@ -38,6 +38,7 @@ enum agx_dbg {
    AGX_DBG_DEQP  = BITFIELD_BIT(1),
    AGX_DBG_NO16  = BITFIELD_BIT(2),
    AGX_DBG_DIRTY  = BITFIELD_BIT(3),
+   AGX_DBG_PRECOMPILE  = BITFIELD_BIT(4),
 };
 
 struct agx_device {