Initial commit of Mach-O object format.

Initial commit of Mach-O object format.

Contributed by: Henryk Richter <henryk.richter@comlab.uni-rostock.de>

This adds 3 object format keywords: macho, macho32, macho64.  These work in
the same way as elf, elf32, and elf64.  The object format is still a work
in progress; amongst other things it does not yet support full cross-section
references (othersym1-othersym2), dynamic linking, or GAS input syntax.  We
will continue to improve and work on these features in the near future.

svn path=/trunk/yasm/; revision=1732