Auto merge of #37714 - alexcrichton:builtins-hidden, r=nikomatsakis

rustc: Flag all builtins functions as hidden

When compiling compiler-rt you typically compile with `-fvisibility=hidden`
which to ensure that all symbols are hidden in shared objects and don't show up
in symbol tables. This is important for these intrinsics being linked in every
crate to ensure that we're not unnecessarily bloating the public ABI of Rust
crates.

This should help allow the compiler-builtins project with Rust-defined builtins
start landing in-tree as well.

diff --cc src/librustc_trans/declare.rs
index 662e3bec66db874cad34a8920558e7bb0962f928,9ed3bfa04a8926fc8cc1dfca6c0f7f59327cf02c..7d6a672077a07c888fe4456224b9e25334a12f0c
--- 1/src/librustc_trans/declare.rs
--- 2/src/librustc_trans/declare.rs
+++ b/src/librustc_trans/declare.rs
@@@ -25,9 -26,9 +26,10 @@@ use rustc::ty
  use abi::{Abi, FnType};
  use attributes;
  use context::CrateContext;
 +use common;
  use type_::Type;
  use value::Value;
+ use syntax::attr;
  
  use std::ffi::CString;