keep using poll as fast path and only use fcntl as fallback

this minimizes the amount of syscalls performed during startup

library/std/src/lib.rs

@@ -239,6 +239,7 @@
 #![feature(dropck_eyepatch)]
 #![feature(exhaustive_patterns)]
 #![feature(intra_doc_pointers)]
+#![feature(label_break_value)]
 #![feature(lang_items)]
 #![feature(let_chains)]
 #![feature(linkage)]