cmd/go: fix handling of build tags for standard commands Otherwise if I add '+build !race' to e.g. src/cmd/fix/reflect_test.go, it does not work.
cmd/go: fix handling of build tags for standard commands
Otherwise if I add '+build !race' to e.g. src/cmd/fix/reflect_test.go,
it does not work.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/6642044

gnanderson/go,dsriram/golang,tv42/old-go,gnanderson/go,pombredanne/go-deleteme,apparentlymart/golang,gnanderson/go,apparentlymart/golang,tv42/old-go,dsriram/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,apparentlymart/golang,pombredanne/go-deleteme,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,dsriram/golang,gnanderson/go,dsriram/golang,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,gnanderson/go,dsriram/golang,dsriram/golang,gnanderson/go,gnanderson/go,tv42/old-go,dsriram/golang