Fix broken disassembly of floating point immediates on big endian hosts

[huth]

Disassembling single floating points with immediate values currently gives wrong results on big endian hosts (like s390x), e.g.:

 ./cstool/cstool m68k40 'f2 3c 44 22 40 49 0e 56'
  0  f2 3c 44 22 40 49 0e 56  fadd.s     #0.000000, fp0

While it should be (like on x86):

 ./cstool/cstool m68k40 'f2 3c 44 22 40 49 0e 56'
  0  f2 3c 44 22 40 49 0e 56  fadd.s     #3.141500, fp0

The problem is that these single float values are supposed to be stored in the 32-bit "simm" field of struct cs_m68k_op (see e.g. the printing of M68K_FPU_SIZE_SINGLE in printAddressingMode() in M68KInstPrinter.c), but currently the immediate is only written to the 64-bit "imm" field of the union in cs_m68k_op. This works on little endian systems, since the least significant bytes overlap in the union there. For example, let's assume that the value 0x01020304 gets written to "imm":

 04 03 02 01 00 00 00 00    uint64_t imm
 xx xx xx xx xx xx xx xx    double dimm;
 xx xx xx xx .. .. .. ..    float simm;

But on big endian hosts, the important bytes do not overlap, so "simm" is always zero there:

 00 00 00 00 01 02 03 04    uint64_t imm
 xx xx xx xx xx xx xx xx    double dimm;
 xx xx xx xx .. .. .. ..    float simm;

To fix the problem, let's always set "simm" explicitly, this works on both, big endian and little endian hosts.

Thanks to Michal Schulz for his initial analysis of the problem (in #1710) and to Travis Finkenauer for providing an easy example to reproduce the issue (in #1931).

Closes: #1710

[Rot127]

LGTM. Though what is about case 0x5 and the default case?
Aren't they broken as well? If yes, it would be great if you could quickly fix them here as well.

[huth]

@Rot127 : case 0x05 is fine since it is about double floats that have 64 bit length - so this aliases perfectly well with the "imm" that gets written by get_ea_mode_op(). And the "default" case does not seem to use get_ea_mode_op() at all, i.e. it does not set the "imm" field at all, so I assume that's not affected.

[Rot127]

@kabeor Please take a look.

[kabeor]

Looks cool, thanks!