cmd/compile: don't use fast32/64 map functions for aggregates
Under register ABI, aggregates like [2]uint32 is passed
differently than a uint64. For now, don't use the fast version
of the map functions for non-trivial aggregates.
GOEXPERIMENT=regabi,regabiargs can now pass make.bash, modulo
staleness checks.
TODO: maybe find some way to use the fast functions. Maybe
unsafe-cast to uint32/64 then call the map function. But need to
make the type checker happy.