Switch to rust-lang-nursery/compiler-builtins

This commit migrates the in-tree `libcompiler_builtins` to the upstream version
at https://github.com/rust-lang-nursery/compiler-builtins. The upstream version
has a number of intrinsics written in Rust and serves as an in-progress rewrite
of compiler-rt into Rust. Additionally it also contains all the existing
intrinsics defined in `libcompiler_builtins` for 128-bit integers.

It's been the intention since the beginning to make this transition but
previously it just lacked the manpower to get done. As this PR likely shows it
wasn't a trivial integration! Some highlight changes are:

* The PR rust-lang-nursery/compiler-builtins#166 contains a number of fixes
  across platforms and also some refactorings to make the intrinsics easier to
  read. The additional testing added there also fixed a number of integration
  issues when pulling the repository into this tree.

* LTO with the compiler-builtins crate was fixed to link in the entire crate
  after the LTO process as these intrinsics are excluded from LTO.

* Treatment of hidden symbols was updated as previously the
  `#![compiler_builtins]` crate would mark all symbol *imports* as hidden
  whereas it was only intended to mark *exports* as hidden.

src/bootstrap/bootstrap.py

@@ -583,7 +583,7 @@ class RustBuild(object):
                                (self.get_toml('jemalloc') or
                                 self.get_mk('CFG_JEMALLOC_ROOT'))))]
         run(["git", "submodule", "update",
-             "--init"] + submodules,
+             "--init", "--recursive"] + submodules,
             cwd=self.rust_root, verbose=self.verbose)
         run(["git", "submodule", "-q", "foreach", "git",
              "reset", "-q", "--hard"],