Greatly reduce amount of debuginfo compiled for bootstrap itself

Rather than compiling rustbuild and all its dependencies with
`debuginfo=2`, this compiles dependencies without debuginfo and
rustbuild with `debuginfo=1`. On my laptop, this brings compile times
down from ~1:20 to ~1:05.

src/bootstrap/bootstrap.py

@@ -921,10 +921,9 @@ class RustBuild(object):
         env["LIBRARY_PATH"] = os.path.join(self.bin_root(True), "lib") + \
             (os.pathsep + env["LIBRARY_PATH"]) \
             if "LIBRARY_PATH" in env else ""
+
         # preserve existing RUSTFLAGS
         env.setdefault("RUSTFLAGS", "")
-        env["RUSTFLAGS"] += " -Cdebuginfo=2"
-
         build_section = "target.{}".format(self.build)
         target_features = []
         if self.get_toml("crt-static", build_section) == "true":