target-arm: A64: Add [UF]RSQRTE (reciprocal root estimate)

target-arm: A64: Add [UF]RSQRTE (reciprocal root estimate)

This adds support for [UF]RSQRTE instructions. It utilises the existing
NEON helpers with some changes. The changes include an explicit passing
of fpstatus (so the correct one is used between arm32 and aarch64),
denormilzation, more correct error handling and also proper scaling of
the fraction going into the estimate.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Message-id: 1394822294-14837-25-git-send-email-peter.maydell@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>