This is a follow up to 841924c9,
where `-fllvm` was explicitly removed from GhcStage1HcOpts for
perf-cross.

After removing -fllvm from GhcStage1HcOpts, it should be removed from
SRC_HC_OPTS as well, because SRC_HC_OPTS are added to every Haskell
compilation. That's what this patch does.

BuildFlavour bench-cross (added in ddf79ebf),
is probably never used. But for consistency, also use -fllvm here, for
building stage2 and the libraries.

Reviewed by: austin

Differential Revision: https://phabricator.haskell.org/D1228

This allows easier diffing of different BuildFlavours, including
`mk/flavours/validate.mk`.

Reviewed By: bgamari, austin

Differential Revision: https://phabricator.haskell.org/D1050