cmd/go: allow output in non-existent directory

When 'go build' is given an output path with -o, if the output path
ends with a path separator, always treat it as a directory.

Fixes #41313

Change-Id: I9a9c25448abfcd6297ad973f5ed2025b2568a4a7
GitHub-Last-Rev: 20a19bd63a2779a2c94b0efdf86146ffd551293c
GitHub-Pull-Request: golang/go#41314
Reviewed-on: https://go-review.googlesource.com/c/go/+/253821


Run-TryBot: Jay Conrod <jayconrod@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Trust: Bryan C. Mills <bcmills@google.com>
Trust: Jay Conrod <jayconrod@google.com>