FORTIFY_SOURCE: strcat / strncat optimize

__strcat_chk and __strncat_chk are slightly inefficient,
because they end up traversing over the same memory region
two times.

This change optimizes __strcat_chk / __strncat_chk so they
only access the memory once. Although I haven't benchmarked these
changes, it should improve the performance of these functions.

__strlen_chk - expose this function, even if -D_FORTIFY_SOURCE
isn't defined. This is needed to compile libc itself without
-D_FORTIFY_SOURCE.

Change-Id: Id2c70dff55a276b47c59db27a03734d659f84b74

libc/include/string.h

@@ -49,6 +49,7 @@ extern char*  strchr(const char *, int) __purefunc;
 extern char*  strrchr(const char *, int) __purefunc;
 
 extern size_t strlen(const char *) __purefunc;
+extern size_t __strlen_chk(const char *, size_t);
 extern int    strcmp(const char *, const char *) __purefunc;
 extern char*  strcpy(char* __restrict, const char* __restrict);
 extern char*  strcat(char* __restrict, const char* __restrict);
@@ -207,8 +208,6 @@ size_t strlcat(char* __restrict dest, const char* __restrict src, size_t size) {
     return __strlcat_chk(dest, src, size, bos);
 }
 
-extern size_t __strlen_chk(const char *, size_t);
-
 __BIONIC_FORTIFY_INLINE
 size_t strlen(const char *s) {
     size_t bos = __bos(s);