Improve math benchmark infrastructure

Improve support for math function benchmarking.  This patch adds
a feature that allows accurate benchmarking of traces extracted
from real workloads.  This is done by iterating over all samples
rather than repeating each sample many times (which completely
ignores branch prediction and cache effects).  A trace can be
added to existing math function inputs via
"## name: workload-<name>", followed by the trace.

        * benchtests/README: Describe workload feature.
        * benchtests/bench-skeleton.c (main): Add support for
        benchmarking traces from workloads.