i386: Hide __old_readdir64 [BZ #18822]

Hide internal __old_readdir64 function to allow direct access within
libc.so and libc.a without using GOT nor PLT.

	[BZ #18822]
	* sysdeps/unix/sysv/linux/i386/olddirent.h (__old_readdir64):
	Add libc_hidden_proto.
	* sysdeps/unix/sysv/linux/i386/readdir64.c (__old_readdir64):
	Add libc_hidden_def.