Enable compiler warnings in stage0

[alexcrichton]

It's been awhile since we had any regressions in lint, and I've started to really hurt by having all lints allow-by-default in stage0 only to catch the bugs 15 minutes later during stage2.

It'd be awesome if we could allow warnings in stage0 by default now. If there's a bug with lint we'd have to wait till the next snapshot for the bug to go away, but we'd still be guaranteed no lints in stage1+

[pnkfelix]

In case anyone else is curious, it is not too hard to override choices like this from the make invocation itself, without editing any of the makefiles themselves (changes that you would have to remember to revert before commiting/merging/etc), in order to select the old -Awarnings in stage 0 and thus quiet down the builds (which have been noisy in recent days due to the deprecated_owned_vector warning that is unrecognized in stage0).

The way I do it is:

make -j8 --environment-overrides WFLAGS_ST0=-Awarnings