examples/embedding: Fix example so it compiles again.
examples/embedding: Fix example so it compiles again.

There were a few changes that had broken this example, specifically
2cdf1d25f59409b6130c0e8b6cf50300aed2d7e6 removed file.c from ports/unix.
And (at least for MacOS) mp_state_ctx must be placed in the BSS with
-fno-common so it is visible to the linker.

Signed-off-by: Santeri Paavolainen <f140dca0ecb379e9a45eb422eb71472290284331@iki.fi>

bvernoux/micropython,henriknelson/micropython,bvernoux/micropython,adafruit/circuitpython,adafruit/circuitpython,bvernoux/micropython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,henriknelson/micropython,henriknelson/micropython,henriknelson/micropython,bvernoux/micropython,bvernoux/micropython,henriknelson/micropython