Update reference of rlibc crate to compiler-builtins crate

src/libcore/lib.rs

@@ -24,7 +24,7 @@
 //!   often generated by LLVM. Additionally, this library can make explicit
 //!   calls to these functions. Their signatures are the same as found in C.
 //!   These functions are often provided by the system libc, but can also be
-//!   provided by the [rlibc crate](https://crates.io/crates/rlibc).
+//!   provided by the [compiler-builtins crate](https://crates.io/crates/compiler_builtins).
 //!
 //! * `rust_begin_panic` - This function takes four arguments, a
 //!   `fmt::Arguments`, a `&'static str`, and two `u32`'s. These four arguments