turn off generational gc mode by default.

in the current implementation, `bin/mruby -e "puts GC.generational_mode"`
would result `true`.

currently the Generational Mode is based on Tri-color infrastructure and
experimental, it's promising, but it's might not our current default
choice.

in theory, the Tri-color GC offers controllable latency, but longer the
GC cycle, so more memory would be consumed overally; Generational GC
shortens the GC cycle, so less memory consumption, offers longer latency
but better overall performance.

but in the current implementation, GC will traverse all the heap slots
in the sweep phase, which downgrade the performance of Generational GC.
we might have to make more optimization on this before making it
default.