thumb: incorrect S bit (setflags == !InITBlock())

[pistol]

Example:

>>> darm.disasm_thumb(0x18cc).__str__()
'add r4, r1, r3'
>>> darm.disasm_thumb(0x18cc)
Darm(instr=I_ADD, instr_type=T_THUMB_3REG, cond=C_AL, Rd=r4, Rn=r1, Rm=r3, rotate=0)

objdump reports this instead as adds (S bit set).

Unlike for A32, S bit is determined by setflags == !InITBlock() according to A.8.8.4 T1.

The caller is responsible for knowing InITBlock().

darm should expose S as one of B_INVLD, B_SET, B_UNSET, B_IT where B_IT is a new state, signaling the caller of the special check required.