Add powf benchmark

As for expf/exp2f benchmark, this patch add two benchmark for powf:
one which measures thoughput and one which measures latency.

The input data is based on a reduced trace from 2.3 billion samples
extracted from specpu2017 521.wrf_r benchmark.

Test: ran 32-bit and 64-bit x86 tests on host
Change-Id: Id8943d90dd5452146e55fb75708daaf7bf0e25fa