util: Fix util_fast_pow/exp2/log2.

util: Fix util_fast_pow/exp2/log2.

- Use a lookup table for log2.

- Compute (float) (1 << ipart) by tweaking with the exponent directly to
avoid integer overflow and float conversion.

- Also table negative exponents to avoid float division and branching.

- Implement util_fast_exp as function of util_fast_exp2.

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Cherry-picked from gallium-0.2: 8415d06d90a197e16554dab98d160334fd9f9f93

This fixes some pow() glitches seen in fslight.c, spectex.c, etc.

Conflicts:
       src/gallium/auxiliary/util/u_math.h