gallivm: unify sin and cos implementation

gallivm: unify sin and cos implementation

The (complicated!) math is all identical, there's just minimal differences how
sign bit is calculated plus there's an additional subtraction for the argument
going into the polynomial for cos.
The logic stays 100% the same (with a small exception, sign bit calculation for
sin is minimally simplified, applying sign mask after xoring the arguments
instead of applying it to each argument).

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>