runtime: fix trigger for concurrent GC

Adjust triggergc so that we trigger when we have used 7/8
of the available heap memory. Do first collection when we
exceed 4Mbytes.

Change-Id: I467b4335e16dc9cd1521d687fc1f99a51cc7e54b
Reviewed-on: https://go-review.googlesource.com/3149


Reviewed-by: Austin Clements <austin@google.com>