cmd/go: fix and re-enable build_trimpath test

The test was comparing a binary built from a list of files to a test
build from a named package. That should not (and did not) work. The
test now compares two binaries built the same way in different
directories.

Also add a portion of the test for GOPATH and fix the gccgo portion of
the test (verified manually).

Fixes #35435

Change-Id: I2535a0011c9d97d2274e5550ae277302dbb91e6f
Reviewed-on: https://go-review.googlesource.com/c/go/+/208234


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