Create an eventfd object with EFD_CLOEXEC flag, to avoid leaking fi…

…le descriptor to child processes

The file descriptor `irq_notify_fd` does not have the close-on-exec (`FD_CLOEXEC`) flag so it possibly leaks to child processes.
This pull request adds an `EFD_CLOEXEC` flag to the second argument of the `exentfd(2)` call, which tells it to set an `FD_CLOEXEC` flag on the new file descriptor.

lib/system/linux/irq.c

@@ -219,7 +219,7 @@ int metal_linux_irq_init(void)
 
 	memset(&irqs, 0, sizeof(irqs));
 
-	irq_notify_fd = eventfd(0, 0);
+	irq_notify_fd = eventfd(0, EFD_CLOEXEC);
 	if (irq_notify_fd < 0) {
 		metal_log(METAL_LOG_ERROR,
 			  "Failed to create eventfd for IRQ handling.\n");