Remove one use of sprintf in ada-lang.c (print_it_exception)

This is just a minor cleanup of an ARI violation.  As this affects
code that is normally unreachable (you'd need a runtime where
the unit providing the exception hook has been compiled without
debugging information), I tested this manually by commenting out
the code just above, thus forcing this part of the code to be
exercised.

gdb/ChangeLog:

        * ada-lang.c (print_it_exception): Avoid use of sprintf.

Tested on x86_64-linux.

gdb/ada-lang.c

@@ -10783,7 +10783,7 @@ print_it_exception (enum exception_catchpoint_kind ex, struct breakpoint *b)
 		 just replace the exception name by the generic string
 		 "exception" - it will read as "an exception" in the
 		 notification we are about to print.  */
-	      sprintf (exception_name, "exception");
+	      memcpy (exception_name, "exception", sizeof ("exception"));
 	    }
 	  /* In the case of unhandled exception breakpoints, we print
 	     the exception name as "unhandled EXCEPTION_NAME", to make