Add nacl.bash, the NaCl version of all.bash.
It's a separate script because it builds a variant of package syscall
with a large zip file embedded in it, containing all the input files
needed for tests.

Disable various tests new since the last round, mostly the ones using os/exec.

Fixes #7945.

LGTM=dave
R=golang-codereviews, remyoudompheng, dave, bradfitz
CC=golang-codereviews
https://golang.org/cl/100590044

        Fixes #3699.

R=golang-dev, r, rsc
CC=golang-dev
https://golang.org/cl/6272048

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5986063

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

R=bradfitz, ality, r, r, iant, mikioh.mikioh
CC=golang-dev
https://golang.org/cl/5615058

R=r
CC=golang-dev
https://golang.org/cl/1982049

R=adg, r, PeterGo
CC=golang-dev
https://golang.org/cl/1942044

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

R=rsc, dho, r
CC=golang-dev
https://golang.org/cl/194045

this change.

R=rsc
CC=golang-dev
https://golang.org/cl/171048