avoid xnack, and -fp32-denormals, and +fp64-denormals options for
amdgpu / radeonsi driver. these options don't work with llvm 13,
and generate warnings for all GL apps.

mesa 21 has entirely different code here.

merge mesa 19.1.7 into MesaLib.old.

initial import of Mesa 19.1.7 into MesaLib.old.

import the existing Mesa 10.x sources as MesaLib.old while we
handle the upgrade to Mesa 18.x.