This CL makes it possible to run make.bash with
GOOS and GOARCH set to something other than
the native host GOOS and GOARCH.
As part of the CL, the tool directory moves from bin/tool/
to pkg/tool/goos_goarch where goos and goarch are
the values for the host system (running the build), not
the target. pkg/ is not technically appropriate, but C objects
are there now tool (pkg/obj/) so this puts all the generated
binaries in one place (rm -rf $GOROOT/pkg cleans everything).
Including goos_goarch in the name allows different systems
to share a single $GOROOT on a shared file system.
Fixes#2920.
R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5645093
factor out environment variable checks.
infer $GOROOT etc during build if not set.
it's still necessary to set them for yourself
to use the standard Makefiles.
when running all.bash, don't recompile all the
go packages in run.bash, since make.bash already did.
R=r
CC=golang-dev
https://golang.org/cl/609042
