Hide __chmod and __mkdir [BZ #18822]

Hide __chmod and __mkdir [BZ #18822]

Hide internal __chmod and __mkdir functions to allow direct access
within libc.so and libc.a without using GOT nor PLT.

	[BZ #18822]
	* include/sys/stat.h (__chmod): Add libc_hidden_proto.
	(__mkdir): Likewise.
	* io/chmod.c (__chmod): Add libc_hidden_def.
	* io/mkdir.c (__mkdir): Likewise.
	* sysdeps/mach/hurd/chmod.c (__chmod): Likewise.
	* sysdeps/mach/hurd/mkdir.c (__mkdir): Likewise.
	* sysdeps/unix/sysv/linux/generic/chmod.c (__chmod): Likewise.
	* sysdeps/unix/sysv/linux/generic/mkdir.c (__mkdir): Likewise.