Use sysroot crates maximally in rustc_codegen_gcc.

This shrinks `compiler/rustc_codegen_gcc/Cargo.lock` quite a bit. The
only remaining dependencies in `compiler/rustc_codegen_gcc/Cargo.lock`
are `gccjit`, `lang_tester`, and `boml`, all of which aren't used in any
other compiler crates.

The commit also reorders and adds comments to the `extern crate` items
so they match those in miri.

Cargo.toml

@@ -23,21 +23,11 @@ default = ["master"]
 
 [dependencies]
 gccjit = "2.1"
-
 # Local copy.
 #gccjit = { path = "../gccjit.rs" }
 
-object = { version = "0.30.1", default-features = false, features = [
-    "std",
-    "read",
-] }
-smallvec = { version = "1.6.1", features = ["union", "may_dangle"] }
-# TODO(antoyo): make tempfile optional.
-tempfile = "3.7.1"
-
 [dev-dependencies]
 lang_tester = "0.8.0"
-tempfile = "3.1.0"
 boml = "0.3.1"
 
 [profile.dev]

src/lib.rs

@@ -24,6 +24,14 @@
 #![deny(clippy::pattern_type_mismatch)]
 #![allow(clippy::needless_lifetimes)]
 
+// Some "regular" crates we want to share with rustc
+extern crate object;
+extern crate smallvec;
+extern crate tempfile;
+#[macro_use]
+extern crate tracing;
+
+// The rustc crates we need
 extern crate rustc_apfloat;
 extern crate rustc_ast;
 extern crate rustc_attr;
@@ -42,8 +50,6 @@ extern crate rustc_middle;
 extern crate rustc_session;
 extern crate rustc_span;
 extern crate rustc_target;
-#[macro_use]
-extern crate tracing;
 
 // This prevents duplicating functions and statics that are already part of the host rustc process.
 #[allow(unused_extern_crates)]