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

src/Make.common

@@ -6,7 +6,7 @@ clean:
 	rm -rf *.o *.a *.[$(OS)] [$(OS)].out $(CLEANFILES)
 
 %.make:
-	(cd $* && gomake install)
+	gomake -C $* install
 
 .PHONY: all clean nuke install coverage test bench testpackage-clean\ 
 	importpath dir

src/pkg/Makefile

@@ -211,19 +211,19 @@ test.dirs: $(addsuffix .test, $(TEST))
 bench.dirs: $(addsuffix .bench, $(BENCH))
 
 %.clean:
-	+cd $* && $(MAKE) clean
+	+$(MAKE) -C $* clean
 
 %.install:
-	+cd $* && $(MAKE) install
+	+$(MAKE) -C $* install
 
 %.nuke:
-	+cd $* && $(MAKE) nuke
+	+$(MAKE) -C $* nuke
 
 %.test:
-	+cd $* && $(MAKE) test
+	+$(MAKE) -C $* test
 
 %.bench:
-	+cd $* && $(MAKE) bench
+	+$(MAKE) -C $* bench	
 
 clean: clean.dirs