fix wrong gadget selection in setRegisters() (#1118)

[bruce30262]

For issue #1118

[zachriggle]

I don't think the detection for int instructions works correctly:

>>> context.arch='amd64'
>>> r=ROP(ELF.from_assembly('syscall; pop rdx; int 0x80; pop rsi; ret'))
...
>>> r.gadgets
{268435456L: Gadget(0x10000000, [u'syscall', u'pop rdx', u'int 0x80'], [u'rdx'], 0x8),
 268435458L: Gadget(0x10000002, [u'pop rdx', u'int 0x80'], [u'rdx'], 0x8),
 268435459L: Gadget(0x10000003, [u'int 0x80'], [], 0x0),
 268435461L: Gadget(0x10000005, [u'pop rsi', u'ret'], [u'rsi'], 0x10),
 268435462L: Gadget(0x10000006, [u'ret'], [], 0x8)}

Would you also mind adding a few tests for that routine, to test those edge cases?

Thanks!

[bruce30262]

@zachriggle I've add the test cases, please take a look