armv7m: Raise correct kind of UsageFault for attempts to execute ARM code

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>

linux-user/main.c

@@ -574,6 +574,7 @@ void cpu_loop(CPUARMState *env)
         switch(trapnr) {
         case EXCP_UDEF:
         case EXCP_NOCP:
+        case EXCP_INVSTATE:
             {
                 TaskState *ts = cs->opaque;
                 uint32_t opcode;