Commit d33e0668 accidentally disabled
varargs support unless long double support is enabled.

Also disable it by default for now until x87 support is more mature.

This commit also abstracts the node types used for fxch/fdup/fpop/ffreep
by using callbacks in the x87_simulator_config.

mode_E is only 10 bytes wide but we should still use the 12byte wide
type_E for better alignment/consistency.