lib/posix-event: `SYS_eventfd2`: Use low-level `fcntl` syscall

lib/posix-event: `SYS_eventfd2`: Use low-level `fcntl` syscall

The eventfd constructor system call `eventfd2` uses the the `fcntl` system
call to setup the file descriptor flag `O_NONBLOCK` during creation. This
commit makes sure that the low-level implementation is directly called so
that there is no dependency to `errno`.

Checkpatch-Ignore: AVOID_EXTERNS
Checkpatch-Ignore: FUNCTION_ARGUMENTS
Signed-off-by: Simon Kuenzer <simon@unikraft.io>
Reviewed-by: Florin Postolache <florin.postolache.of@gmail.com>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #903