runtime: remove g.stackAlloc

Since we're no longer stealing space for the stack barrier array from
the stack allocation, the stack allocation is simply
g.stack.hi-g.stack.lo.

Updates #17503.

Change-Id: Id9b450ae12c3df9ec59cfc4365481a0a16b7c601
Reviewed-on: https://go-review.googlesource.com/36621


Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>