cmd/go: add -trimpath build flag

"go build -trimpath" trims the recorded file paths in the
resulting packages and executables to avoid recording
the names of any local directories. Instead, the files appear
to be stored in directories named either "go/src/..." (for the
standard library) or named after the module or package
in which the files appear.

Fixes #16860.

Change-Id: I433afeeb1fdeea641286b21693fee5e0a66d607e
Reviewed-on: https://go-review.googlesource.com/c/go/+/173345


Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>