[Bug c/64862] printf attribute should accept other string types

[schwab@linux-m68k.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64862

--- Comment #1 from Andreas Schwab <schwab@linux-m68k.org> ---
glibc has this in <wchar.h>:

extern int wprintf (const wchar_t *__restrict __format, ...)
     /* __attribute__ ((__format__ (__wprintf__, 1, 2))) */;