target/arm: use raise_exception_ra for stack limit exception

The sequence cpu_restore_state() + raise_exception() is equivalent to
raise_exception_ra(), so use that instead.  (In this case we never
cared about the syndrome value, because M-profile doesn't use the
syndrome; the old code was just written unnecessarily awkwardly.)

Cc: Richard Henderson <richard.henderson@linaro.org>
Cc: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Jamie Iles <jamie@nuviainc.com>
[PMM: Retain edited version of comment; rewrite commit message]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>