gallivm/llvmpipe: add support for front facing in sysval.
This wires up the front facing value as a sysval, I'd like to
remove the other facing code but I'd need to confirm VMware
don't use it first.
Reviewed-by: 
Marek Olšák <marek.olsak@amd.com>
This wires up the front facing value as a sysval, I'd like to
remove the other facing code but I'd need to confirm VMware
don't use it first.
Reviewed-by: Marek Olšák <marek.olsak@amd.com>