Incorrect registers in x86_64 ret/retf

[hainest]

Work environment

Questions	Answers
System Capstone runs on OS/arch/bits	Ubuntu x86_64
Capstone module affected	x86
Source of Capstone	git clone
Version/git commit	8872be6

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0xcb is decoded as X86_LRETL instead of X86_LRETQ.

./cstool -dar x64 cb
 0  cb                                               retf	
	ID: 351 (retf)
	Prefix:0x00 0x00 0x00 0x00 
	Opcode:0xcb 0x00 0x00 0x00 
	rex: 0x0
	addr_size: 8
	modrm: 0x0
	disp: 0x0
	sib: 0x0
	Registers read: esp ss
	Registers modified: esp eip cs
	Groups: ret not64bitmode

The read set should be [rsp, ss] and the write set [rsp, rip, ss].

See #2789 for details. This can wait until a decision has been made about a new x86 backend (cf. #2505).