runtime: allocate more stack space in CgoCallbackGC test

Allocate enough stack space so that the test will work on a system
that does not support split stacks.

This test is actually not very meaningful for gccgo at present, but it
doesn't hurt to keep running it.

Updates golang/go#20931

Change-Id: I81f5b7d48cd7de828def6ae6180d0b7c12809902
Reviewed-on: https://go-review.googlesource.com/50630
Reviewed-by: Than McIntosh <thanm@google.com>
1 file changed