test: issue16016: use fewer goroutines for gccgo

test: issue16016: use fewer goroutines for gccgo

For https://gcc.gnu.org/PR114453

Change-Id: If41d9fca6288b18ed47b0f21ff224c74ddb34958
Reviewed-on: https://go-review.googlesource.com/c/go/+/574536
Reviewed-by: Than McIntosh <thanm@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>