Today, 'go build -a my/pkg' and 'go install -a my/pkg'
recompile not just my/pkg and all its dependencies that
you wrote but also the standard library packages.
Recompiling the standard library is problematic on
some systems because the installed copy is not writable.

The -a behavior means that you can't use 'go install -a all'
or 'go install -a my/...' to rebuild everything after a Go
release - the rebuild stops early when it cannot overwrite
the installed standard library.

During development work, however, you do want install -a
to rebuild everything, because anything might have changed.

Resolve the conflict by making the behavior of -a depend
on whether we are using a released copy of Go or a devel copy.
In the release copies, -a no longer applies to the standard library.
In the devel copies, it still does.

This is the latest in a long line of refinements to the
"do I build this or not" logic. It is surely not the last.

Fixes #8290.

LGTM=r
R=golang-codereviews, r, tracey.brendan
CC=adg, golang-codereviews, iant
https://golang.org/cl/151730045