windows: Don't link rust_builtin

This library has no shims which are actually needed on Windows now, so translate
that last easy one into Rust and then don't link it at all on Windows.

src/libtest/lib.rs

@@ -872,7 +872,7 @@ fn run_tests<F>(opts: &TestOpts,
 
 #[allow(deprecated)]
 fn get_concurrency() -> usize {
-    match env::var("RUST_TEST_THREADS") {
+    return match env::var("RUST_TEST_THREADS") {
         Ok(s) => {
             let opt_n: Option<usize> = s.parse().ok();
             match opt_n {
@@ -884,10 +884,24 @@ fn get_concurrency() -> usize {
             if std::rt::util::limit_thread_creation_due_to_osx_and_valgrind() {
                 1
             } else {
-                extern { fn rust_get_num_cpus() -> libc::uintptr_t; }
-                unsafe { rust_get_num_cpus() as usize }
+                num_cpus()
             }
         }
+    };
+
+    #[cfg(windows)]
+    fn num_cpus() -> usize {
+        unsafe {
+            let mut sysinfo = std::mem::zeroed();
+            libc::GetSystemInfo(&mut sysinfo);
+            sysinfo.dwNumberOfProcessors as usize
+        }
+    }
+
+    #[cfg(unix)]
+    fn num_cpus() -> usize {
+        extern { fn rust_get_num_cpus() -> libc::uintptr_t; }
+        unsafe { rust_get_num_cpus() as usize }
     }
 }