arm64: use the correct function type for __arm64_sys_ni_syscall

Calling sys_ni_syscall through a syscall_fn_t pointer trips indirect
call Control-Flow Integrity checking due to a function type
mismatch. Use SYSCALL_DEFINE0 for __arm64_sys_ni_syscall instead and
remove the now unnecessary casts.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>