FROMLIST: 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.

Change-Id: Id01f505b333b864477335e87e0c3100fa60be1eb
Link: https://lkml.org/lkml/2019/5/3/690


Signed-off-by: Sami Tolvanen <samitolvanen@google.com>