On Plan 9, sysReserve was ignoring the address hint and allocating
memory wherever it is available.  This causes the new
TestArenaCollision test to fail on 32-bit Plan 9.  We now use the
address hint in the specific case where sysReserve is extending the
process address space at its end, and similarly we contract the
address space in the case where sysFree is releasing memory at
the end.

Fixes #23860

Change-Id: Ia5254779ba8f1698c999832720a88de400b5f91a
Reviewed-on: https://go-review.googlesource.com/94776Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: David du Colombier <0intro@gmail.com>