cmd/go: add a test for -sectcreate in LDFLAGS

cmd/go: add a test for -sectcreate in LDFLAGS

It was already covered by a regex pattern, but it didn't have a test.

To fix the issue for good, added regression test.

Fixes #28832.

Change-Id: I861e3bed92d3b9484fd8671270dbd2e264b10d2d
Reviewed-on: https://go-review.googlesource.com/c/go/+/191311
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>