cmd/compile: omit file:pos for non-existent, permission errors

Omits printing the file:line:column when trying to open either
* non-existent files
* files without permission

Given:
    go tool compile x.go

For either of x.go not existing, or if no read permissions:

* Before:
    x.go:0: open x.go: no such file or directory
    x.go:0: open x.go: permission denied

* After:
    open x.go: no such file or directory
    open x.go: permission denied

While here, noticed an oddity with the Linux builders, that appear
to always be running under root, hence the test for permission errors
with 0222 -W-*-W-*-W- can't pass on linux-amd64 builders.
The filed bug is #38608.

Fixes #36437

Change-Id: I9645ef73177c286c99547e3a0f3719fa07b35cb5
Reviewed-on: https://go-review.googlesource.com/c/go/+/229357


Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>