kbuild: add endianness flag to CHEKCFLAGS

The kernel depends on macros like __BYTE_ORDER__,
__BIG_ENDIAN__ or __LITTLE_ENDIAN__.

OTOH, sparse doesn't know about the endianness of the kernel and
by default uses the same as the machine on which sparse was built.

Ensure that sparse can predefine the macros corresponding to
how the kernel was configured by adding -m{big,little}-endian
to CHECKFLAGS in the main Makefile (and so for all archs).
Also, remove the equivalent done in arch specific Makefiles.

Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>