Add ./x.py check src/{libstd,libtest,rustc}.

This currently only supports a limited subset of the full compilation,
but is likely 90% of what people will want and is possible without
building a full compiler (i.e., running LLVM). In theory, this means
that contributors who don't want to build LLVM now have an easy way to
compile locally, though running tests won't work.

src/bootstrap/bootstrap.py

@@ -602,6 +602,7 @@ class RustBuild(object):
         env["LIBRARY_PATH"] = os.path.join(self.bin_root(), "lib") + \
             (os.pathsep + env["LIBRARY_PATH"]) \
             if "LIBRARY_PATH" in env else ""
+        env["RUSTFLAGS"] = "-Cdebuginfo=2"
         env["PATH"] = os.path.join(self.bin_root(), "bin") + \
             os.pathsep + env["PATH"]
         if not os.path.isfile(self.cargo()):