Improve generic strspn performance

As for strcspn, this patch improves strspn performance using a much
faster algorithm.  It first constructs a 256-entry table based on
the accept string and then uses it as a lookup table for the
input string.  As for strcspn optimization, it is generally at least
10 times faster than the existing implementation on bench-strspn
on a few AArch64 implementations.

Also the string/bits/string2.h inlines make no longer sense, as current
implementation will already implement most of the optimizations.

Tested on x86_64, i686, and aarch64.

	* string/strspn.c (strcspn): Rewrite function.
	* string/bits/string2.h (strspn): Use __builtin_strcspn.
	(__strspn_c1): Remove inline function.
	(__strspn_c2): Likewise.
	(__strspn_c3): Likewise.
	* string/string-inlines.c
	[SHLIB_COMPAT(libc, GLIBC_2_1_1, GLIBC_2_24)] (__strspn_c1): Add
	compatibility symbol.
	[SHLIB_COMPAT(libc, GLIBC_2_1_1, GLIBC_2_24)] (__strspn_c2):
	Likewise.
	[SHLIB_COMPAT(libc, GLIBC_2_1_1, GLIBC_2_24)] (__strspn_c3):
	Likewise.

string/string-inlines.c

@@ -71,4 +71,40 @@ __old_strcspn_c3 (const char *__s, int __reject1, int __reject2,
   return __result;
 }
 compat_symbol (libc, __old_strcspn_c3, __strcspn_c3, GLIBC_2_1_1);
+
+size_t
+__old_strspn_c1 (const char *__s, int __accept)
+{
+  size_t __result = 0;
+  /* Please note that __accept never can be '\0'.  */
+  while (__s[__result] == __accept)
+    ++__result;
+  return __result;
+}
+compat_symbol (libc, __old_strspn_c1, __strspn_c1, GLIBC_2_1_1);
+
+size_t
+__old_strspn_c2 (const char *__s, int __accept1, int __accept2)
+{
+  size_t __result = 0;
+  /* Please note that __accept1 and __accept2 never can be '\0'.  */
+  while (__s[__result] == __accept1 || __s[__result] == __accept2)
+    ++__result;
+  return __result;
+}
+compat_symbol (libc, __old_strspn_c2, __strspn_c2, GLIBC_2_1_1);
+
+size_t
+__old_strspn_c3 (const char *__s, int __accept1, int __accept2,
+		 int __accept3)
+{
+  size_t __result = 0;
+  /* Please note that __accept1 to __accept3 never can be '\0'.  */
+  while (__s[__result] == __accept1 || __s[__result] == __accept2
+	 || __s[__result] == __accept3)
+    ++__result;
+  return __result;
+}
+compat_symbol (libc, __old_strspn_c3, __strspn_c3, GLIBC_2_1_1);
+
 #endif