go/internal/gccgoimporter: change all .gox files to text

go/internal/gccgoimporter: change all .gox files to text

Avoid problems with distro scanning by not using files that look like
object files.

For #48548

Change-Id: I4a06d5690d0870d6253f176e510faa5eebf2d057
Reviewed-on: https://go-review.googlesource.com/c/go/+/352450
Trust: Ian Lance Taylor <iant@golang.org>
Trust: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>