Rollup merge of #94524 - bjorn3:remove_num_cpus, r=Mark-Simulacrum

Remove num_cpus dependency from bootstrap, build-manifest and rustc_s…

…ession

`std::threads::available_parallelism` was stabilized in rust 1.59.

r? ```````````````````````````@Mark-Simulacrum```````````````````````````

Cargo.lock

@@ -221,7 +221,6 @@ dependencies = [
  "getopts",
  "ignore",
  "libc",
- "num_cpus",
  "once_cell",
  "opener",
  "pretty_assertions",
@@ -249,7 +248,6 @@ dependencies = [
  "anyhow",
  "flate2",
  "hex 0.4.2",
- "num_cpus",
  "rayon",
  "serde",
  "serde_json",
@@ -4242,7 +4240,6 @@ name = "rustc_session"
 version = "0.0.0"
 dependencies = [
  "getopts",
- "num_cpus",
  "rustc_ast",
  "rustc_data_structures",
  "rustc_errors",