[v3] add c_compatibility headers

[Benjamin Kosnik <bkoz at redhat dot com>]

This patch adds the c_compatibility headers that are necessary to get
--enable-cheaders=c to work with newlib. Two additional patches are also
necessary, one for gcc/ginclude and one for newlib itself.

The newlib patches should be integrated shortly.

The gcc patch is posted for discussion only. Eventually something like
this should be done, but as the new "C" header model is pretty new,
there's no point in applying it now, as it will break the old model.
(There are ways to do both but this patch doesn't address that.)

Nits:

1) c_compatibility headers have avoided C99 and wchar_t issues. Nuff
said. Some improvement on this front would be necessary for (say) glibc
to switch over to this model.

2) I fudged the math.h C99 issues. Nuff said. 

The build process is relatively straightforward. I'll post a message to
the libstdc++ list on how to build a newlib-based cross with this
activated.

I intend to document the "C" header issues. Hopefully that will follow
on monday.

best,
benjamin

tested x86/linux
tested x86/linux x powerpc-eabisim --enable-cheaders=c, patches

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