turn off generational gc mode by default.

in the current implementation, `bin/mruby -e "puts GC.generational_mode"` would result `true`.

currently GC infrastructure is Tri-color, and 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 less latency, but longer the GC cycle, so more memory would be consumed overally; Generational GC offers a shorter GC cycle, so less memory consumption, and offers 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.