target/arm: Add v8M stack checks on exception entry

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