teach sparse about __BYTE_ORDER__ & __ORDER_{BIG,LITTLE}_ENDIAN__

teach sparse about __BYTE_ORDER__ & __ORDER_{BIG,LITTLE}_ENDIAN__

Some macros, structures definitions, ... are endianness specific.
This is generaly done with the help of some header files but these
headers often need information from the compiler via the macro
__BYTE_ORDER__, itself being defined to __ORDER_BIG_ENDIAN__ or
__ORDER_LITTLE_ENDIAN__.

Without these defines, sparse's pre-processor may interpret things
differently than the compiler would do which may hide errors or
warn on code that is compiled out.

Fix this by letting sparse predefine these macros like compilers do.

Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Acked-by: Christopher Li <sparse@chrisli.org>