Hide __readv and __writev [BZ #18822]

Hide __readv and __writev [BZ #18822]

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

	[BZ #18822]
	* include/sys/uio.h (__readv): Add libc_hidden_proto.
	(__writev): Likewise.
	* misc/readv.c (__readv): Add libc_hidden_def.
	* misc/writev.c (__writev): Likewise.
	* sysdeps/posix/readv.c (__readv): Likewise.
	* sysdeps/posix/writev.c (__writev): Likewise.
	* sysdeps/unix/sysv/linux/readv.c: Include <sys/uio.h>.
	(__readv): Likewise.
	* sysdeps/unix/sysv/linux/writev.c: Include <sys/uio.h>.
	(__writev): Likewise.