stddef.h, [C++] wchar_t vs size_t

[Jason Merrill]

As brent says, wchar_t is supposed to be a keyword in C++, so it
should not be defined in the header.  We used to fudge that a bit in
g++ so that headers which do define it would still compile; apparently
some of that support has since been removed, without actually doing
away with the __wchar_t hack.

The short answer is, everything in init_decl_processing needs to be
updated to put things in std:: rather than the global namespace when
-fhonor-std.

Jason