Responding to Elliot's suggestions

* changed from uname -s to chpl_platform.py on the argument that
  any futzing with uname will already have taken place there (i.e.,
  why duplicate that logic if corner cases come up in the future).
  One slight weirdness here is that I choose --target (since that's
  where the code is running) even though the .prediff script itself
  is running on the host (i.e., we're basically assuming that host
  will equal target for this test, but that's already true of the
  test).

* changed from the old way I was querying physical memory to
  'sysctl hw.memsize'.  For whatever reason, this was the second
  suggestion on the page that google took me to, so once the
  first suggestion worked, I never bothered looking at the second.
  Fixed up the math based on this change.

* removed some debugging comments (left in on purpose to aid future
  debugging, but I'm not opposed to removing them either).