hadrian: Ensure that -Werror is passed when compiling the RTS.

Previously the `+werror` transformer would only pass `-Werror` to GHC,
which does not ensure that the same is passed to the C compiler when
building the RTS. Arguably this is itself a bug but for now we will just
work around this by passing `-optc-Werror` to GHC.

I tried to enable `-Werror` in all C compilations but the boot libraries
are something of a portability nightmare.