Also fix the interaction between -buildmode and -shared.
It's okay for -shared to change the default build mode,
but it's not okay for it to silently override an explicit -buildmode=exe.

Change-Id: Id40f93d140cddf75b19e262b3ba4856ee09a07ba
Reviewed-on: https://go-review.googlesource.com/10315Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>