target/arm: Assert thumb pc is aligned

target/arm: Assert thumb pc is aligned

Misaligned thumb PC is architecturally impossible.
Assert is better than proceeding, in case we've missed
something somewhere.

Expand a comment about aligning the pc in gdbstub.
Fail an incoming migrate if a thumb pc is misaligned.

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