Events: use eventfd() instead of syscall(SYS_eventfd) if possible. This fixes --with-file-aio support on systems that lack eventfd() syscall, notably aarch64 Linux. The syscall(SYS_eventfd) may still be necessary on systems that have eventfd() syscall in the kernel but lack it in glibc, e.g. as seen in the current CentOS 5 release.