rustbuild: Relax assertions about stage0

This allows bootstrapping new platforms immediately in stage0

src/bootstrap/build/mod.rs

@@ -155,8 +155,7 @@ impl Build {
                                         &compiler, host);
                 }
                 Rustc { stage: 0 } => {
-                    assert!(target.target == self.config.build,
-                            "only have one stage0 compiler");
+                    // nothing to do...
                 }
                 Rustc { stage } => {
                     compile::assemble_rustc(self, stage, target.target);

src/bootstrap/build/step.rs

@@ -194,7 +194,6 @@ impl<'a> Step<'a> {
     pub fn deps(&self, build: &'a Build) -> Vec<Step<'a>> {
         match self.src {
             Source::Rustc { stage: 0 } => {
-                assert!(self.target == build.config.build);
                 Vec::new()
             }
             Source::Rustc { stage } => {