restore attribute((const)) to pthread_self and errno location decls

revert commit a603a75a.

as a result of commit 1c84c999 this is
now safe, assuming an interpretation of the somewhat-underspecified
attribute((const)) consistent with real-world usage.

include/errno.h

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

include/netdb.h

@@ -115,6 +115,9 @@ struct protoent *getprotobynumber (int);
  || (defined(_XOPEN_SOURCE) && _XOPEN_SOURCE+0 < 700)
 struct hostent *gethostbyname (const char *);
 struct hostent *gethostbyaddr (const void *, socklen_t, int);
+#ifdef __GNUC__
+__attribute__((const))
+#endif
 int *__h_errno_location(void);
 #define h_errno (*__h_errno_location())
 #define HOST_NOT_FOUND 1

include/pthread.h

@@ -79,6 +79,9 @@ int pthread_detach(pthread_t);
 _Noreturn void pthread_exit(void *);
 int pthread_join(pthread_t, void **);
 
+#ifdef __GNUC__
+__attribute__((const))
+#endif
 pthread_t pthread_self(void);
 
 int pthread_equal(pthread_t, pthread_t);