Further improve string bench timing

Further improve the timings of the string benchmarks.  Ensure most take
between 1 and 4 seconds to improve accuracy.  Overall time taken increases
by 35%.  Tested on AArch64.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>

	* benchtests/bench-math-inlines.c: Increase iterations.
	* benchtests/bench-memcmp.c: Likewise.
	* benchtests/bench-rawmemchr.c: Likewise.
	* benchtests/bench-strcmp.c: Likewise.
	* benchtests/bench-strcpy_chk.c: Likewise.
	* benchtests/bench-string.h (INNER_LOOP_ITERS8): Add define.
	(INNER_LOOP_ITERS_MEDIUM): Increase iterations.
	(INNER_LOOP_ITERS_SMALL): Likewise.
	* benchtests/bench-strncat.c: Increase iterations.
	* benchtests/bench-strncmp.c: Increase iterations.
	* benchtests/bench-strncpy.c: Reduce iterations for wide strings.
	* benchtests/bench-strrchr.c: Increase iterations.
	* benchtests/bench-strstr.c: Keep iterations unchanged.
	* benchtests/bench-strtod.c: Increase iterations.