Remove __libc_readv and __libc_writev function names.

Continuing the removal of __libc_* function names that are no longer
used anywhere, this patch removes the __libc_readv and __libc_writev
names.

Tested for x86_64 that stripped installed shared libraries are
unchanged by the patch.

	* include/sys/uio.h (__libc_readv): Remove declaration.
	(__libc_writev): Likewise.
	* misc/readv.c (__libc_readv): Rename to __readv.
	(__readv): Do not define as alias.
	(readv): Define as alias of __readv.
	* misc/writev.c (__libc_writev): Rename to __writev.
	(__writev): Do not define as alias.
	(writev): Define as alias of __writev.
	* sysdeps/posix/readv.c (__libc_readv): Rename to __readv.
	(__readv): Do not define as alias.
	(readv): Define unconditionally as alias of __readv.
	* sysdeps/posix/writev.c (__libc_writev): Rename to __writev.
	(__writev): Do not define as alias.
	(writev): Define unconditionally as alias of __writev.
	* sysdeps/unix/syscalls.list (readv): Do not define __libc_readv
	name.
	(writev): Do not define __libc_writev name.