Plan 9 won't let brk shrink the data segment if it's shared with
other processes (which it is in the go runtime).  So we keep track
of the notional end of the segment as it moves up and down, and
call brk only when it grows.

Corrects CL 94776.

Updates #23860.
Fixes #24013.

Change-Id: I754232decab81dfd71d690f77ee6097a17d9be11
Reviewed-on: https://go-review.googlesource.com/97595Reviewed-by: David du Colombier <0intro@gmail.com>
Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>