Simplify and speedup strstr/strcasestr first match

Looking at the benchtests, both strstr and strcasestr spend a lot of time
in a slow initialization loop handling one character per iteration.
This can be simplified and use the much faster strlen/strnlen/strchr/memcmp.
Read ahead a few cachelines to reduce the number of strnlen calls, which
improves performance by ~3-4%.  This patch improves the time taken for the
full strstr benchtest by >40%.

	* string/strcasestr.c (STRCASESTR): Simplify and speedup first match.
	* string/strstr.c (AVAILABLE): Likewise.

(cherry picked from commit 284f42bc)