Log Message:
lang/go: Set GOMAXPROCS according to MAKE_JOBS.

Even though we set MAKE_JOBS_SAFE=no in go-module.mk (not really sure
why, wish we had a comment giving a reason!), golang itself may run
multiple threads internally up to the number of CPUs it finds unless
we sternly instruct it not to abuse resources it wasn't offered by
setting GOMAXPROCS.