benchtests: Enable BENCHSET to run subset of tests

benchtests: Enable BENCHSET to run subset of tests

This patch adds BENCHSET variable to benchtests/Makefile in order to
provide the capability to run a list of subsets of benchmark tests, ie;

    make bench BENCHSET="bench-pthread bench-math malloc-thread"

This helps users to benchmark specific glibc area

ChangeLog:

        * benchtests/Makefile:Add BENCHSET to allow subsets of
        benchmarks to be run.
        * benchtests/README: Add documentation for: Running subsets of
        benchmarks.

Signed-off-by: Victor Rodriguez <victor.rodriguez.bahena@intel.com>
Signed-off-by: Icarus Sparry <icarus.w.sparry@intel.com>
Reviewed-By: Siddhesh Poyarekar <siddhesh@sourceware.org>