How to document many darwin-specific, undocumented target macros

[Mike Stump]

On Jun 25, 2012, at 6:48 AM, Steven Bosscher wrote:
> In fact darwin does follow the
> naming convention, the only difference is that it wraps the section
> name in a segment label (always "__DWARF__") and adds some flags
> (always "regular,debug"). I would have expected there to be a way to
> change a section name in a target-specific way (something like a
> targetm.asm_out.adjust_section_name to transform ".debug_blah" to
> "__DWARF,__debug_blah,regular,debug") but it seems such a mechanism
> doesn't exist. Maybe add a hook for this?

A section name transform hook does seem cleaner to me.  Additionally, this could be used to aid portability of using the section selection across darwin and non-darwin targets, so I think it has value beyond a cleaner interface.