Also call runtime.GC before exit to ensure
that the profiler picks up all allocations.

Fixes #10537.

Change-Id: Ibfbfc88652ac0ce30a6d1ae392f919df6c1e8126
Reviewed-on: https://go-review.googlesource.com/9261Reviewed-by: Dave Cheney <dave@cheney.net>
Reviewed-by: Minux Ma <minux@golang.org>
Run-TryBot: Minux Ma <minux@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>