Refactor run_host_only to have the proper effect.

Previously it was set to true when we didn't run HOSTS steps.

Author: Mark-Simulacrum

src/bootstrap/builder.rs

@@ -149,8 +149,8 @@ impl StepDescription {
 
         // Determine the targets participating in this rule.
         let targets = if self.only_hosts {
-            if build.config.run_host_only {
-                &[]
+            if !build.config.run_host_only {
+                return; // don't run anything
             } else {
                 &build.hosts
             }

src/bootstrap/config.rs

@@ -347,7 +347,7 @@ impl Config {
         config.keep_stage = flags.keep_stage;
 
         // If --target was specified but --host wasn't specified, don't run any host-only tests.
-        config.run_host_only = flags.host.is_empty() && !flags.target.is_empty();
+        config.run_host_only = !(flags.host.is_empty() && !flags.target.is_empty());
 
         let toml = file.map(|file| {
             let mut f = t!(File::open(&file));