Faster MULT32_32_Q31 for ARM.

Uses a C implementation with a 32*32 => 64 multiplication, which
 ARM has.
Speeds up decoding of a 64 kbps test file by 0.5MHz on an ARM7TDMI
 and 1.0MHz on an ARM9TDMI.
0.2% speedup on a 96 kbps enc+dec test on a Cortex A8.

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