armv7m: set CFSR.UNDEFINSTR on undefined instructions

When we take an exception for an undefined instruction, set the
appropriate CFSR bit.

Signed-off-by: Michael Davidsaver <mdavidsaver@gmail.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 1485285380-10565-7-git-send-email-peter.maydell@linaro.org
[PMM: tweaked commit message, comment]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>