hurd: keep only required PLTs in ld.so

We need NO_RTLD_HIDDEN because of the need for PLT calls in ld.so.
See Roland's comment in
https://sourceware.org/bugzilla/show_bug.cgi?id=15605
"in the Hurd it's crucial that calls like __mmap be the libc ones
instead of the rtld-local ones after the bootstrap phase, when the
dynamic linker is being used for dlopen and the like."

We used to just avoid all hidden use in the rtld ; this commit switches to
keeping only those that should use PLT calls, i.e. essentially those defined in
sysdeps/mach/hurd/dl-sysdep.c:

__assert_fail
__assert_perror_fail
__*stat64
_exit

This fixes a few startup issues, notably the call to __tunable_get_val that is
made before PLTs are set up.

sysdeps/mach/hurd/_exit.c

@@ -51,5 +51,7 @@ _exit (int status)
   _hurd_exit (W_EXITCODE (status, 0));
 }
 libc_hidden_def (_exit)
+#ifndef NO_RTLD_HIDDEN
 rtld_hidden_def (_exit)
+#endif
 weak_alias (_exit, _Exit)