travis: Switch unix stackless build to use clang.
travis: Switch unix stackless build to use clang.

To test a different compiler, other than gcc.

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