hurd: Make sure dl-sysdep.c defines proper symbol names

	* sysdeps/mach/hurd/dl-sysdep.c (check_no_hidden): New macro.
	(__open, __close, __libc_read, __libc_write, __writev, __libc_lseek64,
	__mmap, __fxstat64, __xstat64, __access, __access_noerrno, __getpid,
	__getcwd, __sbrk, __strtoul_internal, _exit, abort): Use check_no_hidden
	to make sure that these symbols are defined.