The flag is already set inside planner.get_current_block(). Doing it
twice isn't making it better..

The extrusion speed was wrong due to a not high enough precision of
esteps to XY steps, therefore now the target float values are used to
calculate the ratio between XY movement and extrusion speed.
The e_speed_multiplier8 was replaced by an absolute multiplier called
abs_adv_steps_multiplier8, therefore one multiplication and bitshift can
be saved inside the stepper ISR. Due to this, also extruder_advance_k is
better suited inside the planner and not the stepper files any more.

As noted by https://github.com/MarlinFirmware/Marlin/pull/4971#issuecomment-252503295

・Fix a comment about timer

.) long to int: Extruder stalls at 10kHz / 20kHz step limits with long.
.) Take the delta_adv_steps calculation out of the step_loops loop. Wasted calculation performance if done inside.
.) >> 2 replaced by 3: Is divide by 8. Reason: Timer 0 runs at 16/8=2MHz, Timer 1 at 16/64=0.25MHz. ==> 2/0.25=8.

Probably needed for long straight lines when using high microstepping

Remove a setting proccess of E direction in advance_isr()

Also the extruder stepper ISR has to keep an eye on step_loops count.