benchtests: Allow selecting baseline for compare_string.py

This patch allows one to provide the function name using an optional
-base option to compare all other functions against.  This is useful
when pitching one implementation of a string function against
alternatives.  In the absence of this option, comparisons are done
against the first ifunc in the list.

	* benchtests/scripts/compare_strings.py (main): Add an
	optional -base option.
	(process_results): New argument base_func.