py/asmbase: Revert removal of clearing of label offsets for native emit.
py/asmbase: Revert removal of clearing of label offsets for native emit.

The assembler back-end for most architectures needs to know if a jump is
backwards in order to emit optimised machine code, and they do this by
checking if the destination label has been set or not.  So always reset
label offsets to -1 (this reverts partially the previous commit, with some
minor optimisation for the if-logic with the pass variable).

ryannathans/micropython,trezor/micropython,selste/micropython,swegener/micropython,bvernoux/micropython,bvernoux/micropython,adafruit/circuitpython,ryannathans/micropython,MrSurly/micropython,bvernoux/micropython,pozetroninc/micropython,pozetroninc/micropython,bvernoux/micropython,adafruit/micropython,infinnovation/micropython,blazewicz/micropython,infinnovation/micropython,trezor/micropython,tobbad/micropython,dmazzella/micropython,blazewicz/micropython,pozetroninc/micropython,adafruit/circuitpython,tralamazza/micropython,adafruit/circuitpython,ryannathans/micropython,pozetroninc/micropython,dmazzella/micropython,henriknelson/micropython,selste/micropython,swegener/micropython,adafruit/micropython,infinnovation/micropython,tobbad/micropython,pozetroninc/micropython,kerneltask/micropython,kerneltask/micropython,trezor/micropython,selste/micropython,tralamazza/micropython,pramasoul/micropython,trezor/micropython,pramasoul/micropython,pfalcon/micropython,kerneltask/micropython,henriknelson/micropython,henriknelson/micropython,MrSurly/micropython,swegener/micropython,swegener/micropython,blazewicz/micropython,blazewicz/micropython,bvernoux/micropython,pfalcon/micropython,pfalcon/micropython,infinnovation/micropython,pfalcon/micropython,adafruit/micropython,dmazzella/micropython,henriknelson/micropython,pramasoul/micropython,MrSurly/micropython,henriknelson/micropython,kerneltask/micropython,infinnovation/micropython,tobbad/micropython,swegener/micropython,ryannathans/micropython,adafruit/micropython,pfalcon/micropython,tobbad/micropython,selste/micropython,pramasoul/micropython,tobbad/micropython,MrSurly/micropython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,MrSurly/micropython,trezor/micropython,kerneltask/micropython,selste/micropython,adafruit/micropython,dmazzella/micropython,pramasoul/micropython,ryannathans/micropython,tralamazza/micropython,blazewicz/micropython,tralamazza/micropython