Slightly faster C_MULC for ARMv4.

Reorder register usage to take advantage of early termination on
 multiplications and reorder a load instruction to hide its
 latency on ARM9.
Speeds up decoding of a 64 kbps test file by 0.1MHz on an ARM7TDMI
 and 0.2MHz on an ARM9TDMI.

Signed-off-by: Timothy B. Terriberry <tterribe@xiph.org>