target/arm: Clean excReturn bits when tail chaining

target/arm: Clean excReturn bits when tail chaining

The TailChain() pseudocode specifies that a tail chaining
exception should sanitize the excReturn all-ones bits and
(if there is no FPU) the excReturn FType bits; we weren't
doing this.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190416125744.27770-14-peter.maydell@linaro.org