cmd/compile: add -env key=value flag

cmd/compile: add -env key=value flag

This flag is not terribly useful with the go command, which will pass
all environment variables through to subprocesses it invokes,
but it can be useful in other build systems, notably blaze and bazel,
to pass compiler-debugging variables like GOSSAFUNC through to
the compiler.

We have been maintaining this as a patch against Google's internal
toolchain for many years, and it has proven useful in those non-go-command
contexts.

Change-Id: Ic123193319f3c838a694eda2575347c516b85ac7
Reviewed-on: https://go-review.googlesource.com/c/go/+/507977
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>