Currently, CLONE_VFORK is used without much regard to the stack. This
is dangerous, because anything the child does to the stack is visible
to the parent. For example, if the compiler were to reuse named stack
slots (which it currently doesn't do), it would be easy for the child
running in the same stack frame as the parent to corrupt local
variables that the parent then depended on. We're not sure of anything
specific going wrong in this code right now, but it is at best a
ticking time bomb.

CLONE_VFORK can only safely be used if we ensure the child does not
execute in any of the active stack frames of the parent. This commit
implements this by arranging for the parent to return immediately from
the frame the child will operate in, and for the child to never return
to the frame the parent will operate in.

Fixes #20732.

Change-Id: Iad5b4ddc2b994c082bd278bfd52ef53bd38c037f
Reviewed-on: https://go-review.googlesource.com/46173
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>