core/conv: implement optimized Viterbi decoder

Add a separate, faster convolution decoding implementation for rates
up to N=4 and constraint lengths of K=5 and K=7, which covers the
most GSM code uses. The decoding algorithm exploits the symmetric
structure of the Viterbi add-compare-select (ACS) operation - commonly
known as the ACS butterfly. This shift-register optimization can be
found in the well-known text by Dave Forney.

Forney, G.D., "The Viterbi Algorithm," Proc. of the IEEE, March 1973.

Implementation is non-architecture specific and improves performance on
x86 as well as ARM processors. Existing API is unchanged with optimized
code being called internally for supported codes.

The original code was relicensed under GPLv2-or-later with permission
of copyright holder - Tom Tsou.

Change-Id: I74d355274b4176a7d924f91ef3c96912ce338fb2

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