Make profiler_builtins #![no_core] instead of just #![no_std]

This crate doesn't contain any actual Rust code; it's just C/C++ code built and
packaged in a Rust-friendly way.

Author: Zalathar

library/profiler_builtins/src/lib.rs

@@ -1,11 +1,12 @@
 // tidy-alphabetical-start
 #![allow(internal_features)]
+#![feature(no_core)]
 #![feature(profiler_runtime)]
 #![feature(staged_api)]
 // tidy-alphabetical-end
 
 // Other attributes:
-#![no_std]
+#![no_core]
 #![profiler_runtime]
 #![unstable(
     feature = "profiler_runtime_lib",