M profile doesn't implement ARM, and the architecturally required
behaviour for attempts to execute with the Thumb bit clear is to
generate a UsageFault with the CFSR INVSTATE bit set.  We were
incorrectly implementing this as generating an UNDEFINSTR UsageFault;
fix this.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>