Rollup merge of #30365 - tamird:update-valgrind, r=pnkfelix

Since `darwin` is really `apple-darwin`, the valgrind-rpass tests were not actually being run with valgrind on mac before. Also, the `HOST` check was completely wrong.

r? @alexcrichton

src/test/run-pass-valgrind/exit-flushes.rs

@@ -8,6 +8,9 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// ignore-macos this needs valgrind 3.11 or higher; see
+// https://github.com/rust-lang/rust/pull/30365#issuecomment-165763679
+
 use std::env;
 use std::process::{exit, Command};