Fix setting custom baudrates on linux/ppc{64,64le} machines

serialposix.py's PlatformSpecific._set_special_baudrate uses the
TCGETS2/TCSETS2 ioctls on linux to set custom baud rates, but the
correct ioctl number and constants are not present for ppc.

Fix this by defining the required constants when running on a ppc
machine, as is already done for mips.

Signed-off-by: Shawn Anastasio <shawn@anastas.io>