Adds following instructions:

vabsdub: Vector Absolute Difference Unsigned Byte
vabsduh: Vector Absolute Difference Unsigned Halfword
vabsduw: Vector Absolute Difference Unsigned Word
Signed-off-by: NSandipan Das <sandipandas1990@gmail.com>
[ use ISA300 define. Drop etype ]
Signed-off-by: NNikunj A Dadhania <nikunj@linux.vnet.ibm.com>
Reviewed-by: NRichard Henderson <rth@twiddle.net>
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>