Expose android_set_abort_message().

Removes the leading underscores from __android_set_abort_message() and
moves its declaration into a public header file.

Bug: 17059126
Change-Id: I470c79db47ec783ea7a54b800f8b78ecbe7479ab

libc/private/libc_logging.h

@@ -69,8 +69,6 @@ struct abort_msg_t {
   char msg[0];
 };
 
-void __android_set_abort_message(const char* msg);
-
 //
 // Formats a message to the log (priority 'fatal'), then aborts.
 //