Running these tests on the system stack is problematic because they
allocate Ps, which are large enough to overflow the system stack if
they are stack-allocated. It used to be necessary to run these tests
on the system stack because they were written in C, but since this is
no longer the case, we can fix this problem by simply not running the
tests on the system stack.

This also means we no longer need the hack in one of these tests that
forces the allocated Ps to escape to the heap, so eliminate that as
well.

Change-Id: I9064f5f8fd7f7b446ff39a22a70b172cfcb2dc57
Reviewed-on: https://go-review.googlesource.com/9923Reviewed-by: Rick Hudson <rlh@golang.org>
Run-TryBot: Austin Clements <austin@google.com>