builder: pass GOHOSTOS and GOHOSTARCH to build
builder: pass GOHOSTOS and GOHOSTARCH to build

R=rsc
CC=golang-dev
http://codereview.appspot.com/2759041

apparentlymart/golang,h8liu/golang,dsriram/golang,pombredanne/go-deleteme,dsriram/golang,tv42/old-go,dsriram/golang,tv42/old-go,tv42/old-go,gnanderson/go,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,h8liu/golang,apparentlymart/golang,ssrl/go,ssrl/go,h8liu/golang,tv42/old-go,h8liu/golang,apparentlymart/golang,gnanderson/go,gnanderson/go,dsriram/golang,pombredanne/go-deleteme,h8liu/golang,pombredanne/go-deleteme,dsriram/golang,tv42/old-go,dsriram/golang,tv42/old-go,h8liu/golang,apparentlymart/golang,ssrl/go,pombredanne/go-deleteme,ssrl/go,apparentlymart/golang,dsriram/golang,h8liu/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,gnanderson/go,ssrl/go,apparentlymart/golang,ssrl/go,apparentlymart/golang,ssrl/go,gnanderson/go,gnanderson/go