Remove reference to NOT_USING_KLIBC and cygwin.

The reference to NOT_USING_KLIBC appears to be the only one
in our codebase. This change also removes some cygwin specific
retry logic - all other supported platforms in this section
of the code should support mapping at an offset.

Note that i've reversed the sense of the check, we always sysconf
since that's recommended practice.

Change-Id: Ib985fb665193d7a07a282f7092cd77c0bc508a66