Handle host_machine.system() 'ios' and 'tvos' the same way as 'darwin'

Handle host_machine.system() 'ios' and 'tvos' the same way as 'darwin'

Despite not being documented in Meson's list of canonical system names,
Meson does accept 'ios' mostly a synonym for darwin.

By using 'ios' instead of darwin, it allows distinguishing between the
two in the cases where that is necessary. Therefore, within dav1d, allow
using the 'ios' name as alias for 'darwin' for system name, to allow
using cross files that does this distinction.

meson itself also allows 'tvos' in addition to 'ios' in the internal
`is_darwin()` function, as such all 3 are handled the same here.