env.MkEnv was computing the full gcc command line to report as
$GOGCCFLAGS in "go env" output, which meant running gcc (or clang)
multiple times to discern which flags are available.
We also set $GOGCCFLAGS in the environment, but nothing actually uses
that as far as I can tell - it was always intended only for debugging.
Move GOGCCFLAGS to env.ExtraEnvVars, which displayed in "go env"
output but not set in child processes and not computed nearly as
often.

The effect is that trivial commands like "go help" or "go env GOARCH"
or "go tool -n compile" now run in about 0.01s instead of 0.1s,
because they no longer run gcc 4 times each.

go test -short cmd/go drops from 81s to 44s (and needs more trimming).

The $GOROOT/test suite drops from 92s to 33s, because the number of
gcc invocation drops from 13,336 to 0.

Overall, all.bash drops from 5m53s to 4m07s wall time.

Change-Id: Ia85abc89e1e2bb126b933aff3bf7c5f6c0984cd5
Reviewed-on: https://go-review.googlesource.com/73850
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>