aarch64: fix definition of sem_nsems in semid_ds structure

POSIX requires the sem_nsems member to have type unsigned short. we
have to work around the incorrect kernel type using matching
endian-specific padding.

add aarch64 port

This adds complete aarch64 target support including bigendian subarch.

Some of the long double math functions are known to be broken otherwise
interfaces should be fully functional, but at this point consider this
port experimental.

Initial work on this port was done by Sireesh Tripurari and Kevin Bortis.