Adding cygwin to the list of platforms that use /proc/cpuinfo for this test

Cygwin has a /proc/cpuinfo, so should use the same rule as other
platforms.  Unfortunately, it doesn't have a 'siblings' entry
(at least for my laptop), so it doesn't work.  We may have better
luck on the official test platform.  In any case, this is a
step in the right direction.

I've proposed to Greg that we should make the default case
optimistically use /proc/cpuinfo rather than making it an
error case so that other platforms that support /proc/cpuinfo
will work without human intervention (and if they don't work,
a similar amount of developer intervention is required as if
it gets a cleaner error message).