Set the keepCAFs flag (required for GHCi with dynamic libraries) from an
__attribute__((constructor)) function linked to stage 2 ghc if GhcBuildDylibs
is set in mk/build.mk.

The previous hack (setting it from addDLL) didn't work, because a few CAFs
from libHSbase_dyn were evaluated before the Linker was first invoked by
GHCi.

MERGE TO STABLE