target-arm: A64: Add saturating accumulate ops (USQADD/SUQADD)

target-arm: A64: Add saturating accumulate ops (USQADD/SUQADD)

Add the saturating accumulate operations USQADD and SUQADD
to the A64 instruction set. This completes coverage of A64 Neon.
These operations (which are unsigned + signed -> signed and
signed + unsigned -> unsigned) don't exist in the A32/T32
instruction set, so require a complete new set of helper functions.

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