rustbuild: fix host-only rules ignoring targets in dist steps

`arr` is the actual list of targets participating in steps construction,
but due to rust-lang#38468 the hosts array now consists of only the build triple
for the `dist` steps, hence all non-build-triple targets are lost for
the host-only rules.

Fix this by using the original non-shadowed hosts array in `arr`
calculation. This should unbreak the nightly packaging process.

Fixes rust-lang#38637.

Author: xen0n

src/bootstrap/step.rs

@@ -839,14 +839,20 @@ invalid rule dependency graph detected, was a rule added and maybe typo'd?
                 &self.build.config.target
             };
             // Determine the actual targets participating in this rule.
+            // NOTE: We should keep the full projection from build triple to
+            // the hosts for the dist steps, now that the hosts array above is
+            // truncated to avoid duplication of work in that case. Therefore
+            // the original non-shadowed hosts array is used below.
             let arr = if rule.host {
                 // If --target was specified but --host wasn't specified,
-                // don't run any host-only tests
-                if self.build.flags.target.len() > 0 &&
-                   self.build.flags.host.len() == 0 {
+                // don't run any host-only tests. Also, respect any `--host`
+                // overrides as done for `hosts`.
+                if self.build.flags.host.len() > 0 {
+                    &self.build.flags.host[..]
+                } else if self.build.flags.target.len() > 0 {
                     &[]
                 } else {
-                    hosts
+                    &self.build.config.host[..]
                 }
             } else {
                 targets