[3.14] gh-132917: Use /proc/self/status for mem usage info. (GH-133544) (gh-133718)

On Linux, use /proc/self/status for mem usage info.  Using smaps_rollup is quite a lot slower and
we can get the similar info from /proc/self/status.
(cherry picked from commit 751db4e64993b5af98aa1c12cc9d7c7c5882961b)

Co-authored-by: Neil Schemenauer <nas-github@arctrix.com>