cmd/go: change the gccgo 'package m' regression test to run in GOPATH mode

This test is failing in the builders due to the deployed versions of
gccgo not supporting module mode. However, the bug reproduced in
GOPATH mode too, so that mode should be fine for a regression test.

Updates #34358

Change-Id: I954132a96849e80e8783d4de10389fcab7b14af2
Reviewed-on: https://go-review.googlesource.com/c/go/+/196518
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>

src/cmd/go/testdata/script/mod_m.txt → src/cmd/go/testdata/script/gccgo_m.txt

 # It's absurd, but builds with -compiler=gccgo used to fail to build module m.
 # golang.org/issue/34358
 
+env GO111MODULE=off
+
+[short] skip
+
 cd m
 go build
 exists m$GOEXE