[PATCH v2] Add __pure2 to __locale_ctype_ptr(_l)

[Wilco Dijkstra]

The newlib ctype functons are extremely inefficient due to repeatedly
calling __locale_ctype_ptr for every single use of a ctype macro, even
in a tight loop.  Improve this by adding the missing __pure2 attribute so
the pointer can be cached just like in GLIBC, resulting in > 2x speedup
in loops.

I haven't looked in detail at the implementation, but in general you don't
need a function call - a variable, thread-local if required, would be simpler,
even faster and cache automatically.

ChangeLog:
2017-10-31  Wilco Dijkstra  <wdijkstr@arm.com>

        * newlib/libc/include/ctype.h (__locale_ctype_ptr): Add __pure2.
        (__locale_ctype_ptr_l): Likewise.
--
diff --git a/newlib/libc/include/ctype.h b/newlib/libc/include/ctype.h
index 06458cbda47abe1294ebe226a16981fc9f38254d..a85e157a9019132fd7f9eceba690dcde07be2f3c 100644
--- a/newlib/libc/include/ctype.h
+++ b/newlib/libc/include/ctype.h
@@ -66,7 +66,7 @@ extern int toascii_l (int __c, locale_t __l);
 #define _X	0100
 #define	_B	0200
 
-const char *__locale_ctype_ptr (void);
+const char *__locale_ctype_ptr (void) __pure2;
 # define __CTYPE_PTR	(__locale_ctype_ptr ())
 
 #ifndef __cplusplus
@@ -100,7 +100,7 @@ const char *__locale_ctype_ptr (void);
 #endif
 
 #if __POSIX_VISIBLE >= 200809
-const char *__locale_ctype_ptr_l (locale_t);
+const char *__locale_ctype_ptr_l (locale_t) __pure2;
 #define __ctype_lookup_l(__c,__l) ((__locale_ctype_ptr_l(__l)+sizeof(""[__c]))[(int)(__c)])
 
 #define	isalpha_l(__c,__l)	(__ctype_lookup_l(__c,__l)&(_U|_L))