Pass the mcmodel=medium parameter to CC via GHC

Ensure that GHC-driver builds default to mcmodel=medium, so that GHC
passes this default parameter to CC without having to add it to the
compiled project.

Commit e70d4140 does not ensure that all
GHC-built object files have a default model of medium, and will raise an
R_LARCH_B26 overflow error.