mips64: Consolidate Linux sysdep.h

The mips64 Linux syscall macros only differs argument type and
the requirement of sign-extending values on n32.  The headers
are consolidate by parameterizing the arguments with a new type,
__syscall_arg_t, and by defining the ARGIFY for n64.

Also, the generic unix mips64 sysdep is essentially the same,
only the load instruction need to be adjusted depending of the
ABI.

Checked on mips64-linux-gnu and mips64n32-linux-gnu.