MIPS: Fast path for rgb888 to rgb32 conversion

Add a hand-coded MIPS assembler function to convert a stream of rgb888
bytes to an array of 32-bit ARGB values. This is used in QImage for
image conversion, and also in QJpegHandler for decoding JPEG image
scanlines.

Change-Id: Ia74a5ff799b29fad3d4acbfcafe790cd60586d72
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>