Larger stack frames mean nosplit functions use more stack and so the limit
needs to increase.

The change to test/nosplit.go is a bit ugly but I can't really think of a
way to make it nicer.

Change-Id: I2616b58015f0b62abbd62951575fcd0d2d8643c2
Reviewed-on: https://go-review.googlesource.com/16504Reviewed-by: Russ Cox <rsc@golang.org>