I finally remembered I was supposed to be doing this when I had some free time.

Comments?  Anybody want to (help me) fix anything before I commit, or just let 
it fall on the floor later (not a bad option)?  It builds for me on RH 8 with 
or without the --enable-nonportable-atomics option.  With the patch and 
--enable-nonportable-atomics, testatomics does complete successfully.
I'm not patient enough to see if testatomics will complete using the generic 
atomics code.  Even the code at HEAD may take forever AFAICT.

Win32 logic should be in its own atomic/win32/atomics.c; I haven't checked if 
they need some of the default function implementations.  If they do, then 
likely some code needs to move around further.