target-arm: A64: Add SIMD three-different multiply accumulate insns

target-arm: A64: Add SIMD three-different multiply accumulate insns

Add support for the multiply-accumulate instructions from the
SIMD three-different instructions group (C3.6.15):
 * skeleton decode of unallocated encodings and split of
   the group into its three sub-parts
 * framework for handling the 64x64->128 widening subpart
 * implementation of the multiply-accumulate instructions
   SMLAL, SMLAL2, UMLAL, UMLAL2, SMLSL, SMLSL2, UMLSL, UMLSL2,
   UMULL, UMULL2, SMULL, SMULL2

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>