Removing unnecessary instruction from ffi_call_unix64 (#588)

Removing unnecessary instruction from ffi_call_unix64 (#588)

unix64.S's `ffi_call_unix64` looks like it used to take six parameters,
where the sixth said the number of SSE register arguments. However,
currently the function only takes five parameters, and the number of SSE
register arguments is encoded in the `struct register_args *` passed as
the first parameter to `ffi_call_unix64`. This change removes an
instruction that tries to use this missing sixth parameter as the number
of SSE arguments.

This fix should not change any behavior, nor fix any bugs, because a few
instructions later the value moved from %r9d into %eax is overwritten by
the correct value anyway. This change merely makes the code a tad less
confusing, because currently the assembly moves from a register (r9)
whose value is never set.