build: reduce the use of subshells in recursive make
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