armv8: Use __aarch64__ rather than CONFIG_ARM64 in some cases

armv8: Use __aarch64__ rather than CONFIG_ARM64 in some cases

The toolchain sets __aarch64__ for both LE and BE.  In the case of
posix_types.h we cannot reliably use config.h as that will lead to
problems.  In the case of byteorder.h it's clearer to check the EB flag
being set in either case instead.

Cc: David Feng <fenghua@phytium.com.cn>
Signed-off-by: Tom Rini <trini@ti.com>
Amended by Albert ARIBAUD <albert.u.boot@aribaud.net> to
actually remove the config.h include from the posix_types.h
files, with permission from Tom Rini.