x86-64: Update strlen.S to support wcslen/wcsnlen

The difference between strlen and wcslen is byte vs int.  We can
replace pminub and pcmpeqb with pminud and pcmpeqd to turn strlen
into wcslen.

	* sysdeps/x86_64/strlen.S (PMINU): New.
	(PCMPEQ): Likewise.
	(SHIFT_RETURN): Likewise.
	(FIND_ZERO): Replace pcmpeqb with PCMPEQ.
	(strlen): Add SHIFT_RETURN before ret.  Replace pcmpeqb and
	pminub with PCMPEQ and PMINU.
	* sysdeps/x86_64/wcsnlen.S: New file.