target/arm: Migrate v7m.other_sp

target/arm: Migrate v7m.other_sp

In commit abc24d86cc0364f we accidentally broke migration of
the stack pointer value for the mode (process, handler) the CPU
is not currently running as. (The commit correctly removed the
no-longer-used v7m.current_sp flag from the VMState but also
deleted the still very much in use v7m.other_sp SP value field.)

Add a subsection to migrate it again. (We don't need to care
about trying to retain compatibility with pre-abc24d86cc0364f
versions of QEMU, because that commit bumped the version_id
and we've since bumped it again a couple of times.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20180209165810.6668-11-peter.maydell@linaro.org