target-arm: Set privileged bit in TB flags correctly for M profile

M profile ARM cores don't have a CPSR mode field. Set the bit in the
TB flags that indicates non-user mode correctly for these cores.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
