Previously cross-compile.go ran NumCPU builds in parallel, each of
which ran an unrestricted go build using -p NumCPU internally, giving
up to NumCPU^2 concurrent compile processes and enormous load averages.
Pass -p to each go build, sized so the total parallelism is about
NumCPU, sharing the CPUs between however many builds are actually
selected. This can be overridden with the new -build-p flag.
Nick Craig-Wood
3f8df416c8