target-arm: A64: Add saturating int ops (SQNEG/SQABS)

target-arm: A64: Add saturating int ops (SQNEG/SQABS)

This mostly re-uses the existing NEON helpers with an additional two for
the 64 bit case. I also took the opportunity to add TCG_CALL_NO_RWG
options to the helpers as they don't modify globals (saturation flags
are in the CPU Environment).

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>