Add exp and exp2 benchmark

This patch add two benchmarks for both exp and exp2, one which measures
thoughput 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 expf/exp2f benchmark (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