target-arm: A64: add support for BR, BLR and RET insns

target-arm: A64: add support for BR, BLR and RET insns

Implement BR, BLR and RET. This is all of the 'unconditional
branch (register)' instruction category except for ERET
and DPRS (which are system mode only).

Signed-off-by: Alexander Graf <agraf@suse.de>
[claudio: reimplemented on top of new decoder structure]
Signed-off-by: Claudio Fontana <claudio.fontana@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>