cmd/go: allow -o to point to a folder that writes multiple execs

If -o points to a directory that exists then allow multiple
executables to be written to that directory.

Fixes #14295

Change-Id: Ic951e637c70a2ada5e7534bae9a43901a39fe2c5
Reviewed-on: https://go-review.googlesource.com/c/go/+/167679


Run-TryBot: Daniel Theophanes <kardianos@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>