wav writing uses wrong endiannes on Arm Windows

Reading audio output files from IVAS_dec with python on Arm Windows results in an error:

ValueError: File format b'FFIR' not understood. Only 'RIFF', 'RIFX', and 'RF64' supported.

This seems to be caused by lib_util/tinywaveout_c.h incorrectly detecting the endianness of the platform.