arm64: vdso: Add generic syscall stub

As a hack to allow syscalls in the presence of a seccomp filter
that only allows syscalls from the vDSO, add a generic syscall stub
into the vDSO.

Userspace's SIGSYS handler can call this stub as a fallback for
syscalls that it doesn't need to emulate.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>