Fix p{readv,writev}{64} consolidation implementation

This patch fixes the p{readv,writev}{64} consolidation implementation
from commits 4e778151 and af5fdf5a.  Different from pread/pwrite
implementation, preadv/pwritev implementation does not require
__ALIGNMENT_ARG because kernel syscall prototypes define
the high and low part of the off_t, if it is the case, directly
(different from pread/pwrite where the architecture ABI for passing
64-bit values must be in consideration for passsing the arguments).

It also adds some basic tests for preadv/pwritev.

Tested on x86_64, i686, and armhf.

	* misc/Makefile (tests): Add tst-preadvwritev and tst-preadvwritev64.
	* misc/tst-preadvwritev.c: New file.
	* misc/tst-preadvwritev64.c: Likewise.
	* sysdeps/unix/sysv/linux/preadv.c (preadv): Remove SYSCALL_LL{64}
	usage.
	* sysdeps/unix/sysv/linux/preadv64.c (preadv64): Likewise.
	* sysdeps/unix/sysv/linux/pwritev.c (pwritev): Likewise.
	* sysdeps/unix/sysv/linux/pwritev64.c (pwritev64): Likewise.
	* sysdeps/unix/sysv/linux/sysdep.h (LO_HI_LONG): New macro.