Fix FindMatchLength with non-aligned buffers.
Fix FindMatchLength with non-aligned buffers.

The 32-bit buffers are actually rarely 64-bit aligned.
The new solution uses memcmp and is alignment agnostic.
It is also slightly faster.

Change-Id: I863003e9ee4ee8a3eed25b7b2478cb82a0ddbb20

ttyangf/libwebp,webmproject/libwebp,PKRoma/libwebp,ttyangf/libwebp,PKRoma/libwebp,PKRoma/libwebp,ttyangf/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,ttyangf/libwebp,PKRoma/libwebp,ttyangf/libwebp,PKRoma/libwebp