MINOR: tools: add get_exec_path implementation for solaris based systems.

We can use getexecname() which fetches AT_SUN_EXECNAME from the auxiliary
vectors.

src/tools.c

@@ -4821,6 +4821,8 @@ const char *get_exec_path()
 			break;
 		}
 	}
+#elif defined(__sun)
+	ret = getexecname();
 #endif
 	return ret;
 }