eal: introduce portable format attribute

eal: introduce portable format attribute

When using __attribute__((format(...)) on functions, GCC on Windows
assumes MS-specific format string by default, even if the underlying
stdio implementation is ANSI-compliant (either MS Unicersal CRT
or MinGW implementation). Wrap attribute into a macro that forces
GNU-specific format string when using GCC.

Use this new attribute for logging and panic messages in EAL
and for output strings in cmdline library.

Signed-off-by: Dmitry Kozlyuk <dmitry.kozliuk@gmail.com>
Tested-by: William Tu <u9012063@gmail.com>
Tested-by: Pallavi Kadam <pallavi.kadam@intel.com>