Restore simulator more to original version

I realize now the simulator does not want to be an AVR so much
as it wants to be a generic device.  Some AVR has sneaked in, but
my port a while back added more.  It was the wrong direction to go.
Revert more of that.
gcode: fix HEATER_EXTRUDER reference

A rare combination of factors makes this code assume HEATER_EXTRUDER
always exists, when that is not necessarily so.  Add the normal
guard around it.