The whole GOROOT/pkg tree is installed using the GOHOSTOS/GOHOSTARCH
toolchain (installed in GOROOT/pkg/tool/GOHOSTOS_GOHOSTARCH).
The testgo.exe we run during the cmd/go test will be built
for GOOS/GOARCH, which means it will use the GOOS/GOARCH toolchain
(installed in GOROOT/pkg/tool/GOOS_GOARCH).

If these are not the same toolchain, then the entire standard library
will look out of date to testgo.exe (the compilers in those two different
tool directories are built for different architectures and have different
buid IDs), which will cause many tests to do unnecessary rebuilds
and some tests to attempt to overwrite the installed standard library,
which will in turn make it look out of date to whatever runs after the
cmd/go test exits.

Bail out entirely in this case instead of destroying the world.

The changes outside TestMain are checks that might have caught
this a bit earlier and made it much less confusing to debug.

Fixes #22709.
Fixes #22965.

Change-Id: Ibf28fa19e29a1f1b8f17875f446d3474dd04a924
Reviewed-on: https://go-review.googlesource.com/81516
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>