Fix format attribute for printf

Fix format attribute for printf

Since a long time (GCC 4.4?) GCC does support annotating functions
with either the format attribute "gnu_printf" or "ms_printf" to
distinguish between different format string interpretations.

However, it seems like the attribute is ignored for the "printf"
symbol; regardless what the function declaration says, GCC treats
it as "ms_printf". This has become an issue now that mingw-w64
supports using the UCRT instead of msvcrt.dll, and in this case
the stdio functions are declared with the gnu_printf attribute,
and inttypes.h uses the same format specifiers as in GNU mode.

A reproducible example of the problem:

$ cat format.c
__attribute__((__format__ (gnu_printf, 1, 2))) int printf (const char *__format, ...);
__attribute__((__format__ (gnu_printf, 1, 2))) int othername (const char *__format, ...);

void function(void) {
    long long unsigned x = 42;
    othername("%llu\n", x);
    printf("%llu\n", x);
}
$ x86_64-w64-mingw32-gcc -c -Wformat format.c
format.c: In function 'function':
format.c:7:15: warning: unknown conversion type character 'l' in format [-Wformat=]
    7 |     printf("%llu\n", x);
      |               ^
format.c:7:12: warning: too many arguments for format [-Wformat-extra-args]
    7 |     printf("%llu\n", x);
      |            ^~~~~~~~

Note how both functions, printf and othername, are declare with
identical gnu_printf format attributes - GCC does take this into
account for "othername" and doesn't produce a warning, but GCC
seems to disregard the attribute in the printf declaration and
behave as if it was declared as ms_printf.

If the printf function declaration is changed into a static inline
function, the actual attribute used is honored though.

gcc/c-family/ChangeLog:

	PR c/95130
	* c-format.cc: skip default format for printf symbol if
	explicitly declared by prototype.

Signed-off-by: Tomas Kalibera <tomas.kalibera@gmail.com>
Signed-off-by: Jonathan Yong <10walls@gmail.com>