arch: Add a CONFIG_LITTLE_ENDIAN hidden Kconfig option

In preparation for using CONFIG_*_ENDIAN instead of __BYTE_ORDER__, add
a reflection of CONFIG_BIG_ENDIAN that will allow users to conditionally
compile with #ifdef instead of #ifndef while keeping the default case
(little endian) as the first block in the conditional compilation.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>