The basic Getrlimit functionality is already tested in TestRlimit. Use
the Linux-specific test to check whether setting RLIMIT_AS has the
desired effect regarding mmap, as specified in
http://man7.org/linux/man-pages/man2/getrlimit.2.html

Change-Id: I3e2c5769d9144cb4059735709a89a12f42568465
Reviewed-on: https://go-review.googlesource.com/100175
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>