Don't use std:: paths in syntax extensions when compiling a #![no_std] crate

Fixes #16803.
Fixes #14342.
Fixes half of #21827 -- slice syntax is still broken.

src/liballoc/lib.rs

@@ -126,7 +126,8 @@ pub fn oom() -> ! {
 #[doc(hidden)]
 pub fn fixme_14344_be_sure_to_link_to_collections() {}
 
-#[cfg(not(test))]
+// NOTE: remove after next snapshot
+#[cfg(all(stage0, not(test)))]
 #[doc(hidden)]
 mod std {
     pub use core::fmt;