target/arm: Allow explicit writes to CONTROL.SPSEL in Handler mode

In ARMv7M the CPU ignores explicit writes to CONTROL.SPSEL
in Handler mode. In v8M the behaviour is slightly different:
writes to the bit are permitted but will have no effect.

We've already done the hard work to handle the value in
CONTROL.SPSEL being out of sync with what stack pointer is
actually in use, so all we need to do to fix this last loose
end is to update the condition we use to guard whether we
call write_v7m_control_spsel() on the register write.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 1512153879-5291-3-git-send-email-peter.maydell@linaro.org
