While adding support for __HAVE_CPU_COUNTER to sparc64 recently within about
5 minutes, I wondered what other low-hanging fruits there are that I could
collect.

I looked a bit around and found ~0 documentation about those macros. Shouldn't
we have a central list with a short description for each?

For a surrogate I looked at i386, and found these defines that sparc64 does
not have and that I have no idea what they would mean:

sparc does use wscons on krups without defining this, and it works,
so what's this about?
#define __HAVE_NWSCONS

no idea at all:
#define __HAVE_MINIMAL_EMUL

I have an idea and could look details up, but docs would be better:
#define __HAVE_SYSCALL_INTERN

I know what this means, but have no idea why sparc64 does not yet have it -
just noone did it? it's not needed on this arch? (the latter should be 
documented somewhere)
#define __HAVE_RAS

Many of this is probably documented in the mailing list archives, and I'm not
asking for individual explanations for the examples above. What's the right
place/way to document this?

Martin