target/arm: Don't calculate lr in arm_v7m_cpu_do_interrupt() until needed

target/arm: Don't calculate lr in arm_v7m_cpu_do_interrupt() until needed

Move the code in arm_v7m_cpu_do_interrupt() that calculates the
magic LR value down to when we're actually going to use it.
Having the calculation and use so far apart makes the code
a little harder to understand than it needs to be.

Signed-off-by: Peter Maydell <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Edgar E. Iglesias <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Message-id: 1501692241[email protected]