travis: Build qemu-arm with MP_ENDIANNESS_BIG=1 to test bigendian build.
travis: Build qemu-arm with MP_ENDIANNESS_BIG=1 to test bigendian build.

Eventually it would be good to run the full test suite on a big-endian
system, but for now this will help to catch build errors with the
big-endian configuration.

Signed-off-by: Damien George <99e6b749acbfbe2a596df99e91d24d6e1fdbee00@micropython.org>

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