bug#40737: Segfault in arm gcc7, thumb2 builroot, with arm patch

Here is the start of a case to go in tests/movi.c:

#include "test.h"

// Should really test all of the cases seen in
// arm-cpu.c: encode_thumb_immediate()

/* 00000000 00000000 00000000 abcdefgh */
/* 00000000 abcdefgh 00000000 abcdefgh */
/* abcdefgh 00000000 abcdefgh 00000000 */
/* abcdefgh abcdefgh abcdefgh abcdefgh */
/* 1bcdefgh