Update stdarch submodule (to before it switched to const generics)

This also includes a cherry-pick of
ec1461905b
and https://github.com/rust-lang/stdarch/pull/1108 to fix a build
failure.

It also adds a re-export of various macros to the crate root of libstd -
previously they would show up automatically because std_detect was defined
in the same crate.

Cargo.toml

@@ -45,6 +45,8 @@ exclude = [
   # not all `Cargo.toml` files are available, so we exclude the `x` binary,
   # so it can be invoked before the current checkout is set up.
   "src/tools/x",
+  # stdarch has its own Cargo workspace
+  "library/stdarch",
 ]
 
 [profile.release.package.compiler_builtins]