Define _RUST_STAGEN when building rustrt.

This lets us use #ifdefs to determine which stage of the build we happen
to be in, which is useful in the event we need to make changes to rustrt
that are incompatible with the code generated by stage0.

This should help pave the way to completing #6575, which will likely
require changes to type signatures for spawn_fn & glue_fn in rustrt.