benchtests: Add -f/--functions argument

On x86-64, there may be multiple IFUNC implementations for a given
function.  But we may be only interested in a subset of them.  This
patch adds -f/--functions argument to compare a subset of IFUNC
implementations.

	* benchtests/scripts/compare_strings.py (process_results): Add
	funcs argument.  Compare only functions which are selected.
	(main): Check if base function is among selected functions.
	Pass selected functions to process_results.
	(__main__): Add -f/--functions argument.