X86-64: Define LO_HI_LONG to skip pos_h [BZ #20349]

Define LO_HI_LONG to skip pos_h since it is ignored by kernel:

static inline loff_t pos_from_hilo(unsigned long high, unsigned long low)
{
 #define HALF_LONG_BITS (BITS_PER_LONG / 2)
        return (((loff_t)high << HALF_LONG_BITS) << HALF_LONG_BITS) | low;
}

where size of loff_t == size of long.

	[BZ #20349]
	* sysdeps/unix/sysv/linux/x86_64/sysdep.h (LO_HI_LONG): New.