x86 fixes

x86 fixes
Make the emulation environment consider the rex prefix.
Implement and hook in forms of j, jmp, cmp, syscall, movzx
Added a format for an instruction to carry a call to the SE mode syscalls system
Made memory instructions which refer to the rip do so directly
Made the operand size overridable in the microassembly
Made the "ext" field of register operations 16 bits to hold a sparse encoding of flags to set or conditions to predicate on
Added an explicit "rax" operand for the syscall format
Implemented syscall returns.

--HG--
extra : convert_revision : ae84bd8c6a1d400906e17e8b8c4185f2ebd4c5f2