Revert "Remove num_cpus dependency from bootstrap, build-manifest and rustc_session"

This reverts commit 2d854f9c34.

compiler/rustc_session/src/options.rs

@@ -578,7 +578,7 @@ mod parse {
     pub(crate) fn parse_threads(slot: &mut usize, v: Option<&str>) -> bool {
         match v.and_then(|s| s.parse().ok()) {
             Some(0) => {
-                *slot = std::thread::available_parallelism().map_or(1, std::num::NonZeroUsize::get);
+                *slot = ::num_cpus::get();
                 true
             }
             Some(i) => {