target-arm: Ignore attempts to set invalid modes in CPSR

Ignore attempts to set the CPSR mode field to an invalid value.
This is UNPREDICTABLE, but we should not cpu_abort() for things
a malicious guest (or a confused user on the gdbstub interface)
can provoke.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
