eal: add static endianness conversion macros

eal: add static endianness conversion macros

These macros resolve to constant expressions that allow developers to
perform endianness conversion on static/const objects, even outside of
function scope as they do not translate to function calls.

This is most useful for static initializers and constant values (whenever
it has to be performed at compilation time). Run-time endianness conversion
of variable values should keep using rte_*_to_*() calls for best
performance.

Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>