lib: posix: allow eventfd on ARCH_POSIX platform

[TomzBench]

Is your enhancement proposal related to a problem? Please describe.
I would like to be able to use eventfd on native_posix_64 platform for testing purposes.

Describe the solution you'd like
Currently - eventfd is treated as a general library feature and maintained under lib/posix ... And this feature depends on !ARCH_POSIX. However - other socket API do not have this same limitation. ...In the context of zephyr, I think eventfd is more of a network feature as this API is only useful when using zsock_poll (as opposed to the more universal k_poll).

So my proposal is to name space this feature using the same infrastructure as other socket API methods. zsock_eventfd. The feature could be an addon similar to socketpair.

Describe alternatives you've considered
I could use zsock_socketpair - however, this is a more heavy weight solution for my use case, and adds addition prj.conf with extra requirements that should not be necessary for my use case (aka: HEAP_MEM_POOL_SIZE, SOCKETPAIR_BUFFER_SIZE)

Additional context
For history - here is the PR where it was decided to require !ARCH_POSIX, #22863
I hope this can be re-evaluated.

[aescolar]

With the new native_sim (successor of native_posix) eventfd can be used.
Therefore closing as done.

[TomzBench]

great !