[php-src] PR #23825: Fall back to epoll_wait when epoll_pwait2 is unavailable at runtime

Pull Request: https://github.com/php/php-src/pull/23825 Author: bukka HAVE_EPOLL_PWAIT2 only means the libc exports the wrapper, which glibc does since 2.35 regardless of the running kernel. A PHP built on a kernel with epoll_pwait2 and run on one older than 5.11 gets ENOSYS from every Context::wait(), which makes the epoll backend and thus Backend::Auto unusable. This is the normal shape for Docker images, distro packages and static builds, and it also happens under emulation layers that lack the syscall, as seen in #23478. Try epoll_pwait2 first and on ENOSYS or ENOTSUP switch the process to epoll_wait, retrying the current call so the caller never sees the error. The flag is process-wide and atomic so ZTS builds do not race on it. The configure check is unchanged, so the plain link check keeps working when cross-compiling. Alternative to #23478.