Build Mesa with 1 GL driver 10% faster

This compiles marshal_generated.c by forcing -O1 for 1 function and splitting the rest into 7 separate .c files.

Mesa with only radeonsi and 3 state trackers: gl, vdpau, vaapi

Before:

real	1m5.680s
user	12m21.411s
sys	0m41.971s

After:

real	0m59.246s
user	12m3.837s
sys	0m43.346s

CPU: Ryzen 1700X