rustbuild: `make clean` doesn't clean enough

[alexcrichton]

Errors like http://buildbot.rust-lang.org/builders/auto-linux-64-cross-netbsd/builds/462 are happening on the bots, e.g.:

   Compiling bootstrap v0.0.0 (file:///buildslave/rust-buildbot/slave/auto-linux-64-cross-netbsd/build/src/bootstrap)
error: internal compiler error: unexpected panic
note: the compiler unexpectedly panicked. this is a bug.
note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports
thread 'rustc' panicked at 'called `Result::unwrap()` on an `Err` value: Utf8Error { valid_up_to: 0 }', ../src/libcore/result.rs:746
note: Run with `RUST_BACKTRACE=1` for a backtrace.

error: Could not compile `bootstrap`.

To learn more, run the command again with --verbose.

This is #33778 under the covers, but rustbuild's make clean command should blow away the build system directory ahead of time to be more resilient to bugs like this.