Using make -C $* rather than (cd $* ; make) results in a small,
but measurable improvement in build times where compilation is
not the major component. eg.

before - ~/go/src/pkg$ time make
        real	0m1.176s
        user	0m0.639s
        sys	0m0.399s
after - ~/go/src/pkg$ time make
        real	0m0.916s
        user	0m0.571s
        sys	0m0.243s

There are other places in the distribution src/make.common for example
that could also benefit from this change.

R=adg
CC=golang-dev, rsc
https://golang.org/cl/4174055