Don't print bootstrap caching/ensure info unless -vv is passed

Previously, passing `-v` would emit an overwhelming amount of logging:

```
> Std { stage: 1, target: TargetSelection { triple: "x86_64-unknown-linux-gnu", file: None } }
  > Assemble { target_compiler: Compiler { stage: 1, host: TargetSelection { triple: "x86_64-unknown-linux-gnu", file: None } } }
    > Assemble { target_compiler: Compiler { stage: 0, host: TargetSelection { triple: "x86_64-unknown-linux-gnu", file: None } } }
    < Assemble { target_compiler: Compiler { stage: 0, host: TargetSelection { triple: "x86_64-unknown-linux-gnu", file: None } } }
    > Rustc { target: TargetSelection { triple: "x86_64-unknown-linux-gnu", file: None }, compiler: Compiler { stage: 0, host: TargetSelection { triple: "x86_64-unknown-linux-gnu", file: None } } }
      > Std { target: TargetSelection { triple: "x86_64-unknown-linux-gnu", file: None }, compiler: Compiler { stage: 0, host: TargetSelection { triple: "x86_64-unknown-linux-gnu", file: None } } }
        > StartupObjects { compiler: Compiler { stage: 0, host: TargetSelection { triple: "x86_64-unknown-linux-gnu", file: None } }, target: TargetSelection { triple: "x86_64-unknown-linux-gnu", file: None } }
        < StartupObjects { compiler: Compiler { stage: 0, host: TargetSelection { triple: "x86_64-unknown-linux-gnu", file: None } }, target: TargetSelection { triple: "x86_64-unknown-linux-gnu", file: None } }
        c Assemble { target_compiler: Compiler { stage: 0, host: TargetSelection { triple: "x86_64-unknown-linux-gnu", file: None } } }
        > Libdir { compiler: Compiler { stage: 0, host: TargetSelection { triple: "x86_64-unknown-linux-gnu", file: None } }, target: TargetSelection { triple: "x86_64-unknown-linux-gnu", file: None } }
          > Sysroot { compiler: Compiler { stage: 0, host: TargetSelection { triple: "x86_64-unknown-linux-gnu", file: None } } }
          < Sysroot { compiler: Compiler { stage: 0, host: TargetSelection { triple: "x86_64-unknown-linux-gnu", file: None } } }
        < Libdir { compiler: Compiler { stage: 0, host: TargetSelection { triple: "x86_64-unknown-linux-gnu", file: None } }, target: TargetSelection { triple: "x86_64-unknown-linux-gnu", file: None } }
        c Libdir { compiler: Compiler { stage: 0, host: TargetSelection { triple: "x86_64-unknown-linux-gnu", file: None } }, target: TargetSelection { triple: "x86_64-unknown-linux-gnu", file: None } }
        c Sysroot { compiler: Compiler { stage: 0, host: TargetSelection { triple: "x86_64-unknown-linux-gnu", file: None } } }
        c Assemble { target_compiler: Compiler { stage: 0, host: TargetSelection { triple: "x86_64-unknown-linux-gnu", file: None } } }
        > StdLink { compiler: Compiler { stage: 0, host: TargetSelection { triple: "x86_64-unknown-linux-gnu", file: None } }, target_compiler: Compiler { stage: 0, host: TargetSelection { triple: "x86_64-unknown-linux-gnu", file: None } }, target: TargetSelection { triple: "x86_64-unknown-linux-gnu", file: None } }
          c Libdir { compiler: Compiler { stage: 0, host: TargetSelection { triple: "x86_64-unknown-linux-gnu", file: None } }, target: TargetSelection { triple: "x86_64-unknown-linux-gnu", file: None } }
          c Libdir { compiler: Compiler { stage: 0, host: TargetSelection { triple: "x86_64-unknown-linux-gnu", file: None } }, target: TargetSelection { triple: "x86_64-unknown-linux-gnu", file: None } }
        < StdLink { compiler: Compiler { stage: 0, host: TargetSelection { triple: "x86_64-unknown-linux-gnu", file: None } }, target_compiler: Compiler { stage: 0, host: TargetSelection { triple: "x86_64-unknown-linux-gnu", file: None } }, target: TargetSelection { triple: "x86_64-unknown-linux-gnu", file: None } }
      < Std { target: TargetSelection { triple: "x86_64-unknown-linux-gnu", file: None }, compiler: Compiler { stage: 0, host: TargetSelection { triple: "x86_64-unknown-linux-gnu", file: None } } }
... continues for another 150 lines ...
```

This info is occasionally useful when debugging bootstrap itself, but not very useful for figuring
out why a config option was ignored or command wasn't run.  Demote it to `-vv` logging so that `-v`
is more useful.

Author: jyn514

src/bootstrap/builder.rs

@@ -1578,11 +1578,11 @@ impl<'a> Builder<'a> {
                 panic!("{}", out);
             }
             if let Some(out) = self.cache.get(&step) {
-                self.verbose(&format!("{}c {:?}", "  ".repeat(stack.len()), step));
+                self.verbose_than(1, &format!("{}c {:?}", "  ".repeat(stack.len()), step));
 
                 return out;
             }
-            self.verbose(&format!("{}> {:?}", "  ".repeat(stack.len()), step));
+            self.verbose_than(1, &format!("{}> {:?}", "  ".repeat(stack.len()), step));
             stack.push(Box::new(step.clone()));
         }
 
@@ -1605,7 +1605,7 @@ impl<'a> Builder<'a> {
             let cur_step = stack.pop().expect("step stack empty");
             assert_eq!(cur_step.downcast_ref(), Some(&step));
         }
-        self.verbose(&format!("{}< {:?}", "  ".repeat(self.stack.borrow().len()), step));
+        self.verbose_than(1, &format!("{}< {:?}", "  ".repeat(self.stack.borrow().len()), step));
         self.cache.put(step, out.clone());
         out
     }