darwin fix-and-continue support

[Mike Stump]

I've tried to add enough comments for people not versed in macho, uhm, 
code generation to have a fighting chance in understanding some of 
what's going on...  If someone that doesn't understand the details 
wants to try their hand at elaborating the comments even more, I'd be 
happy to answer their questions.  Below is the final version that was 
checked in.

Approved offline by the rs6000 folks and the darwin folks.

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