runtime: make gcTestMoveStackOnNextCall not double the stack

Currently, gcTestMoveStackOnNextCall doubles the stack allocation on
each call because stack movement always doubles the stack. That's
rather unfortunate if you're doing a bunch of stack movement tests in
a row that don't actually have to grow the stack because you'll
quickly hit the stack size limit even though you're hardly using any
of the stack.

Fix this by adding a special stack poison value for
gcTestMoveStackOnNextCall that newstack recognizes and inhibits the
allocation doubling.

Change-Id: Iace7055a0f33cb48dc97b8f4b46e45304bee832c
Reviewed-on: https://go-review.googlesource.com/c/go/+/306672


Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>