target/arm: Make RETA[AB] UNDEF when pauth is not implemented

target/arm: Make RETA[AB] UNDEF when pauth is not implemented

According to the Arm A-profile A64 Instruction Set Architecture,
RETA[AB] should be decoded as UNDEF if the pauth feature is not
implemented.

We got this right in the initial implementation, but accidentally
dropped the feature-check when we converted these insns to
decodetree.

Cc: qemu-stable@nongnu.org
Fixes: 0ebbe9021254f ("target/arm: Convert BRA[AB]Z, BLR[AB]Z, RETA[AB] to decodetree")
Signed-off-by: Solomon Tan <root@wjsota.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20250616171549.59190-1-root@wjsota.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>