Add expf and exp2f benchmark

This patch add two benchmarks for both expf and exp2f, one which
measures thoughput, as default, and one which measures latency. The
latency benchmark works by creating a dependency on the previous
iteration and with a zero multiply to avoid changing the input value.

The input is based on a reduced trace based on 2.4 billion samples
extracted from specpu2017 521.wrf_r benchmark.

Test: ran 32-bit and 64-bit x86 tests on host
Change-Id: Ic0d40cc4977dd6875fb7431146fc38ea0e6e0bc2