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.

include/libc-internal.h

@@ -43,7 +43,7 @@ extern void __libc_thread_freeres (void);
 /* Define and initialize `__progname' et. al.  */
 extern void __init_misc (int, char **, char **) attribute_hidden;
 
-# if IS_IN (rtld) && !defined NO_RTLD_HIDDEN
+# if IS_IN (rtld)
 extern __typeof (__profile_frequency) __profile_frequency attribute_hidden;
 # endif