nrf/mphalport: Remove need for "syntax unified" in mp_hal_delay_us.
nrf/mphalport: Remove need for "syntax unified" in mp_hal_delay_us.

Because it can confuse older versions of gcc.  Instead use the correct
instruction for Thumb vs Thumb-2 (sub vs subs) so the assembler emits the
2-byte instruction.

Related to commit 1aa9ff914194824a78a8b010572ad7083c1bb4ec.

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