We haven't used poisonStack since we switched to 1-bit stack maps
(4d0f3a1c), but the checks are still there. However, nothing prevents
us from genuinely allocating an object at this address on 32-bit and
causing the runtime to crash claiming that it's found a bad pointer.

Since we're not using poisonStack anyway, just pull it out.

Fixes #15831.

Change-Id: Ia6ef604675b8433f75045e369f5acd4644a5bb38
Reviewed-on: https://go-review.googlesource.com/24211
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Keith Randall <khr@golang.org>