Improve generic rawmemchr for targets that don't have an

Improve generic rawmemchr for targets that don't have an
assembler version by tailcalling memchr with the maximum size.
If a target has an optimized memchr this is significantly faster,
if not, then this makes little difference.
Also optimize the special case of zero to use strlen as this is
typically faster than memchr.

	* string/rawmemchr.c (RAWMEMCHR): Use faster memchr/strlen.