Add support for adding x86-64 legacy prefixes (0x66, 0x67) in Ronin::ASM::X86_64::Encoder

[postmodern]

Certain x86-64 instructions assume an operand or memory address size of 64bits, however sometimes a 32bit value is permitted for backwards compatibility. Thus a 0x66 or 0x67 byte is added before the REX prefix or opcode to indicate a 32bit value is used instead of the default 64bit value.

Examples

	mov	10(%ebx), %eax

  401027:	67 8b 43 0a          	mov    0xa(%ebx),%eax

	jmp	*10(%eip)

  401020:	67 ff 25 0a 00 00 00 	jmp    *0xa(%eip)        # 401031 <_start+0x31>

References

• https://wiki.osdev.org/X86-64_Instruction_Encoding#Legacy_Prefixes