Move readv and writev definitions to syscalls.list (bug 14138).

Move readv and writev definitions to syscalls.list (bug 14138).

Continuing the move of syscall definitions to syscalls.list, where the
removal of support for old kernel versions has made this possible,
this patch moves definitions of readv and writev.

The relevant syscalls.list entries were already in
sysdeps/unix/syscalls.list, but to match the C files they needed to
have the names __libc_readv and __libc_writev added.  In fact, I don't
see anything making use of those names - as far as I can tell, these
functions could just be defined as __readv and __writev with aliases
readv and writev.  But cleaning up unnecessary aliases for functions
should be a separate matter from cleaning up unnecessary C syscall
wrappers.

Tested for x86_64.

	[BZ #14138]
	* sysdeps/unix/syscalls.list (readv): Use __libc_readv as strong
	name.
	(writev): Use __libc_writev as strong name.
	* sysdeps/unix/sysv/linux/readv.c: Remove file.
	* sysdeps/unix/sysv/linux/writev.c: Likewise.