Add checks for breaches of the v8M stack limit when the
stack pointer is decremented to push the exception frame
for exception entry.

Note that the exception-entry case is unique in that the
stack pointer is updated to be the limit value if the limit
is hit (per rule R_ZLZG).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20181002163556.10279-7-peter.maydell@linaro.org