posix: relocate Kconfig.eventfd to Kconfig.compat

While event file descriptors (eventfd) are extremely useful, the
API has still not been formalized as part of the POSIX
specification.

Therefore, move this to a separate Kconfig menu for "compat"
(compatibility) features.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>