Auto-detect both if not set, and if GOARCH is not set use GOHOSTARCH.

GOHOSTARCH is used to set the -m32 or -m64 flags for gcc.

This is so that 64-bit can build binaries that run on 32-bit systems.

R=rsc, iant, brainman
CC=golang-dev
https://golang.org/cl/2342045