Cap the number of cpus for Go compiles

When passing "-c" to the Go compiler, any time this value changes, we'd
force all of the Go compiles to rebuild. This could trigger a
substantial portion of the tree to rebuild (anything that transitive
depends on a Go helper tool).

We're running into issues when moving output directories between
multiple GCE machines with different core counts, but are otherwise
identical. This could also hit users moving/mounting disks between
machines, though changes to other host tools can make an impact too.

On my 48-core machine, I get a ~15% benefit from going from -c 1 to -c
48, but also ~12% benefit from going from -c 1 to -c 8. So this will
still let us scale somewhat, but prevent rebuilds when transitioning
between machines that are more likely building Android.