Replace most gcc format attributes by macro GCC_FMT_ATTR (format checking)

Since version 4.4.x, gcc supports additional format attributes.
    __attribute__ ((format (gnu_printf, 1, 2)))
should be used instead of
    __attribute__ ((format (printf, 1, 2))
because QEMU always uses standard format strings (even with mingw32).

The patch replaces format attribute printf / __printf__ by macro
GCC_FMT_ATTR which uses gnu_printf if supported.

It also removes an #ifdef __GNUC__ (not needed any longer).

Cc: Blue Swirl <blauwirbel@gmail.com>
Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
