Make optimize_attribute conditional on libstd

Author: ChrisDenton

crates/as-if-std/src/lib.rs

@@ -2,7 +2,11 @@
 // submodule into the standard library. We try to set this crate up similarly
 // to the standard library itself to minimize the likelihood of issues when
 // updating the `backtrace` crate.
-#![feature(optimize_attribute)]
+#![cfg_attr(
+    // Only used by the gimli backend
+    all(backtrace_in_libstd, unix, not(target_os = "emscripten")),
+    feature(optimize_attribute)
+)]
 #![no_std]
 
 extern crate alloc;