adopt and update libatomic_ops and libgc

[Andy Li]

Hi,

I would like to adopt and update the now orphaned libatomic_ops and
libgc packages, which is a dependency of the neko package I
maintained.

The updated cygport files can be found at:
https://github.com/andyli/libatomic_ops-cygwin
https://github.com/andyli/libgc-cygwin

Note that I've split them into separated packages, since the source of
libgc no longer contains a copy of atomic_ops, and they are versioned
independently as well.

Please review and let me know if there is anything to improve.

Best regards,
Andy