use __attribute__((const)) for errno and pthread_self if __GNUC__ is defined

this is not too ugly and should result in significant code size and
performance improvements for many programs.

include/errno.h

@@ -7,6 +7,9 @@ extern "C" {
 
 #include <bits/errno.h>
 
+#ifdef __GNUC__
+__attribute__((const))
+#endif
 extern int *__errno_location(void);
 #define errno (*__errno_location())