Assembly files with "/vendor/" or "testdata" in their paths were ignored.

Change-Id: I3882ff07eb4426abb9f8ee96f82dff73c81cd61f
GitHub-Last-Rev: 51ae8c324d72a12a059272fcf8568e670bfaf21b
GitHub-Pull-Request: golang/go#31166
Reviewed-on: https://go-review.googlesource.com/c/go/+/170197Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>