Default ABI on Darwin

[Jonathan Perkin <jperkin%joyent.com@localhost>]

Hey all,

I'd like to change the default ABI on Darwin, so that we use ABI=64 on
x86_64 hosts unless the user specifies --abi during bootstrap.  Diff:

  https://gist.github.com/jperkin/ce4c314aa0be2dfb689c

It may be that we want to add a case for ppc64, but I don't have any
hardware to test that on.  Darwin/powerpc developers are welcome to
add one.

This brings us in line with what users expect (I've had a few people
query this previously).  Any objections?

Thanks,

-- 
Jonathan Perkin  -  Joyent, Inc.  -  www.joyent.com