target/arm: Implement MVE VMULL (polynomial)

target/arm: Implement MVE VMULL (polynomial)

Implement the MVE VMULL (polynomial) insn.  Unlike Neon, this comes
in two flavours: 8x8->16 and a 16x16->32.  Also unlike Neon, the
inputs are in either the low or the high half of each double-width
element.

The assembler for this insn indicates the size with "P8" or "P16",
encoded into bit 28 as size = 0 or 1. We choose to follow the
same encoding as VQDMULL and decode this into a->size as MO_16
or MO_32 indicating the size of the result elements. This then
carries through to the helper function names where it then
matches up with the existing pmull_h() which does an 8x8->16
operation and a new pmull_w() which does the 16x16->32.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>