Add LDM / LDMDB JOP gadgets for ARM Thumb2

[face0xff]

Currently, ROPgadget does not include any pattern to detect LDM gadgets in Thumb mode for ARM. Such gadgets can be very useful for JOP in arm32 targets, and Thumb mode vastly increases their availability. LDM gadgets in Thumb2 (32-bit encoding) can also act as nice stack pivots, for instance:

ldm.w r8!, {r0, r1, r2, r3, r4, r5, r6, r7, r8, sb, fp, sp, lr, pc}

On the elf-ARMv7-ls test binary, running ROPgadget with --thumb, the number of gadgets found increases from 131 to 2387, and the number of ldm.w gadgets (--only ldm.w) increases from 0 to 29.

[SweetVishnya]

Hi! Thank you for the patch! Could you also update the tests, please?