py/persistentcode: Fix compilation with load and save both enabled.
py/persistentcode: Fix compilation with load and save both enabled.

With both MICROPY_PERSISTENT_CODE_SAVE and MICROPY_PERSISTENT_CODE_LOAD
enabled the code fails to compile, due to undeclared 'n_obj'.  If
MICROPY_EMIT_NATIVE is disabled there are more errors due to the use of
undefined fields in mp_raw_code_t.

This patch fixes such compilation by avoiding undefined fields.

MICROPY_EMIT_NATIVE was changed to MICROPY_EMIT_MACHINE_CODE in this file
to match the mp_raw_code_t definition.

MrSurly/micropython,MrSurly/micropython,henriknelson/micropython,pfalcon/micropython,bvernoux/micropython,pramasoul/micropython,pozetroninc/micropython,trezor/micropython,selste/micropython,MrSurly/micropython,tobbad/micropython,tobbad/micropython,pfalcon/micropython,henriknelson/micropython,adafruit/circuitpython,pfalcon/micropython,selste/micropython,adafruit/circuitpython,trezor/micropython,adafruit/circuitpython,pramasoul/micropython,pozetroninc/micropython,pramasoul/micropython,tobbad/micropython,pozetroninc/micropython,bvernoux/micropython,henriknelson/micropython,pozetroninc/micropython,bvernoux/micropython,kerneltask/micropython,pfalcon/micropython,tobbad/micropython,bvernoux/micropython,adafruit/circuitpython,trezor/micropython,pramasoul/micropython,kerneltask/micropython,kerneltask/micropython,trezor/micropython,trezor/micropython,pozetroninc/micropython,henriknelson/micropython,henriknelson/micropython,adafruit/circuitpython,MrSurly/micropython,MrSurly/micropython,selste/micropython,pfalcon/micropython,selste/micropython,tobbad/micropython,kerneltask/micropython,pramasoul/micropython,selste/micropython,bvernoux/micropython,kerneltask/micropython,adafruit/circuitpython