Otherwise, we may not have a standard library built for the native "host" target
of the rustc being run.

This isn't an issue for most folks who use x.py dist, which will directly depend
on this. But for x.py build, if we don't properly set target here rustdoc will
not be built.

Currently, there is not a default-on step for generating a rustc for a given
target either, so we will fail to build a rustc as well.