Fixes #11959

This test runs 100 concurrent callbacks from C to Go consuming 100
operating system threads, which at 8mb a piece (the default on linux/arm)
would reserve over 800mb of address space. This would frequently
cause the test to fail on platforms with ~1gb of ram, such as the
raspberry pi.

This change reduces the thread stack allocation to 256kb, a number picked
at random, but at 1/32th the previous size, should allow the test to
pass successfully on all platforms.

Change-Id: I8b8bbab30ea7b2972b3269a6ff91e6fe5bc717af
Reviewed-on: https://go-review.googlesource.com/13731
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Martin Capitanio <capnm9@gmail.com>
Reviewed-by: Minux Ma <minux@golang.org>