Fix pread consolidation on ports that require argument alignment

This patch fixes the __ALIGNMENT_{ARG,COUNT} definition for ports that
define __ASSUME_ALIGNED_REGISTER_PAIRS by including the kernel-features.h
(where it is defined if the case).

This was shown on arm with failing cases:

FAIL: debug/tst-chk1
FAIL: debug/tst-chk2
FAIL: debug/tst-chk3
FAIL: debug/tst-chk4
FAIL: debug/tst-chk5
FAIL: debug/tst-chk6
FAIL: debug/tst-lfschk1
FAIL: debug/tst-lfschk2
FAIL: debug/tst-lfschk3
FAIL: debug/tst-lfschk4
FAIL: debug/tst-lfschk5
FAIL: debug/tst-lfschk6
FAIL: posix/tst-preadwrite
FAIL: posix/tst-preadwrite64

The patches fixes it.  Tested on armhf.

	* sysdeps/unix/sysv/linux/sysdep.h: Include kernel-features.h.