target/arm: Add cpu local variable to exception_return helper

target/arm: Add cpu local variable to exception_return helper

We already call env_archcpu() multiple times within the
exception_return helper function, and we're about to want to
add another use of the ARMCPU pointer. Add a local variable
cpu so we can call env_archcpu() just once.

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