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/private/libc_logging.h

@@ -45,9 +45,6 @@ enum {
   BIONIC_EVENT_MEMSET_BUFFER_OVERFLOW = 80125,
   BIONIC_EVENT_STRCPY_BUFFER_OVERFLOW = 80130,
 
-  BIONIC_EVENT_STRCAT_INTEGER_OVERFLOW = 80200,
-  BIONIC_EVENT_STRNCAT_INTEGER_OVERFLOW = 80205,
-
   BIONIC_EVENT_RESOLVER_OLD_RESPONSE = 80300,
   BIONIC_EVENT_RESOLVER_WRONG_SERVER = 80305,
   BIONIC_EVENT_RESOLVER_WRONG_QUERY = 80310,