mk: Switch rustbuild to the default build system

This commit switches the default build system for Rust from the makefiles to
rustbuild. The rustbuild build system has been in development for almost a year
now and has become quite mature over time. This commit is an implementation of
the proposal on [internals] which slates deletion of the makefiles on
2016-01-02.

[internals]: https://internals.rust-lang.org/t/proposal-for-promoting-rustbuild-to-official-status/4368

This commit also updates various documentation in `README.md`,
`CONTRIBUTING.md`, `src/bootstrap/README.md`, and throughout the source code of
rustbuild itself.

Closes #37858

src/bootstrap/native.rs

@@ -28,7 +28,7 @@ use cmake;
 use gcc;
 
 use Build;
-use util::up_to_date;
+use util::{self, up_to_date};
 
 /// Compile LLVM for `target`.
 pub fn llvm(build: &Build, target: &str) {
@@ -58,6 +58,7 @@ pub fn llvm(build: &Build, target: &str) {
 
     println!("Building LLVM for {}", target);
 
+    let _time = util::timeit();
     let _ = fs::remove_dir_all(&dst.join("build"));
     t!(fs::create_dir_all(&dst.join("build")));
     let assertions = if build.config.llvm_assertions {"ON"} else {"OFF"};
@@ -158,6 +159,17 @@ pub fn test_helpers(build: &Build, target: &str) {
     println!("Building test helpers");
     t!(fs::create_dir_all(&dst));
     let mut cfg = gcc::Config::new();
+
+    // We may have found various cross-compilers a little differently due to our
+    // extra configuration, so inform gcc of these compilers. Note, though, that
+    // on MSVC we still need gcc's detection of env vars (ugh).
+    if !target.contains("msvc") {
+        if let Some(ar) = build.ar(target) {
+            cfg.archiver(ar);
+        }
+        cfg.compiler(build.cc(target));
+    }
+
     cfg.cargo_metadata(false)
        .out_dir(&dst)
        .target(target)