rustbuild: Nicer error for host builds of targets

If a triple is configured only as a target, not a host, then trying to build
that triple with host artifacts would cause a panic. Fail a little nicer
instead.

Closes #36268

src/bootstrap/lib.rs

@@ -959,7 +959,11 @@ impl Build {
     /// Returns the path to the C++ compiler for the target specified, may panic
     /// if no C++ compiler was configured for the target.
     fn cxx(&self, target: &str) -> &Path {
-        self.cxx[target].path()
+        match self.cxx.get(target) {
+            Some(p) => p.path(),
+            None => panic!("\n\ntarget `{}` is not configured as a host,
+                            only as a target\n\n", target),
+        }
     }
 
     /// Returns flags to pass to the compiler to generate code for `target`.