toolchain: Ensure that Kconfig endianness matches the compiler's

In preparation for using CONFIG_(BIG|LITTLE)_ENDIAN exclusively for
endianness-based conditional compilation in the tree, ensure that the
Kconfig CONFIG_(BIG|LITTLE)_ENDIAN options match the compiler's
__BYTE_ORDER__ macro.

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