Added an inline assembly function for the 64 bit multiplication.

Benchmark results on a Pentium II 266 MHz, 512 MB of 24 bit PCM data:

 dd if=/dev/zero bs=64k count=8k |
     time ./test/software_volume 48000:24:2 >/dev/null

Before this patch 22.94s, after this patch 7.24s.