SSSE3 strcpy/stpcpy for x86-64

This patch adds SSSE3 strcpy/stpcpy. I got up to 4X speed up on Core 2
and Core i7.  I disabled it on Atom since SSSE3 version is slower for
shorter (<64byte) data.

string/strncpy.c

@@ -21,11 +21,12 @@
 
 #undef strncpy
 
+#ifndef STRNCPY
+#define STRNCPY strncpy
+#endif
+
 char *
-strncpy (s1, s2, n)
-     char *s1;
-     const char *s2;
-     size_t n;
+STRNCPY (char *s1, const char *s2, size_t n)
 {
   reg_char c;
   char *s = s1;