Improve string benchtest timing

Improve string benchtest timing.  Many tests run for 0.01s which is way too
short to give accurate results.  Other tests take over 40 seconds which is
way too long.  Significantly increase the iterations of the short running
tests.  Reduce number of alignment variations in the long running memcpy walk
tests so they take less than 5 seconds.

As a result most tests take at least 0.1s and all finish within 5 seconds.

	* benchtests/bench-memcpy-random.c (do_one_test): Use medium iterations.
	* benchtests/bench-memcpy-walk.c (test_main): Reduce alignment tests.
	* benchtests/bench-memmem.c (do_one_test): Use small iterations.
	* benchtests/bench-memmove-walk.c (test_main): Reduce alignment tests.
	* benchtests/bench-memset-walk.c (test_main): Reduce alignment tests.
	* benchtests/bench-strcasestr.c (do_one_test): Use small iterations.
	* benchtests/bench-string.h (INNER_LOOP_ITERS): Increase iterations.
	(INNER_LOOP_ITERS_MEDIUM): New define.
	(INNER_LOOP_ITERS_SMALL): New define.
	* benchtests/bench-strpbrk.c (do_one_test): Use medium iterations.
	* benchtests/bench-strsep.c (do_one_test): Use small iterations.
	* benchtests/bench-strspn.c (do_one_test): Use medium iterations.
	* benchtests/bench-strstr.c (do_one_test): Use small iterations.
	* benchtests/bench-strtok.c (do_one_test): Use small iterations.