The test for issue 8945 was marked to only run on gccgo, but there was
no reason for that. It broke for gccgo using GCC 10, because GCC 10
defaults to -fno-common. Make the test run on gc, and split it into
test.go and testx.go to make it work with GCC 10.

The test for issue 9026 used two identical structs which GCC 10 turns
into the same type. The point of the test is not that the structs are
identical, but that they are handled in a particular order. So make
them different.

Updates #8945
Updates #9026

Change-Id: I000fb02f88f346cfbbe5dbefedd944a2c64e8d8e
Reviewed-on: https://go-review.googlesource.com/c/go/+/211217
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>