arm/arm64: Share on_cpus

Now that the previous patches have cleaned up Arm's on_cpus
implementation we can move it to the common lib where riscv
will share it.

Signed-off-by: Andrew Jones <andrew.jones@linux.dev>
Acked-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>