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.

elf/dl-minimal.c

@@ -292,7 +292,9 @@ Inconsistency detected by ld.so: %s: %u: %s%sAssertion `%s' failed!\n",
 		    assertion);
 
 }
+# ifndef NO_RTLD_HIDDEN
 rtld_hidden_weak (__assert_fail)
+# endif
 
 void weak_function
 __assert_perror_fail (int errnum,
@@ -306,7 +308,9 @@ Inconsistency detected by ld.so: %s: %u: %s%sUnexpected error: %s.\n",
 		    __strerror_r (errnum, errbuf, sizeof errbuf));
 
 }
+# ifndef NO_RTLD_HIDDEN
 rtld_hidden_weak (__assert_perror_fail)
+# endif
 #endif
 
 #undef _itoa