We emulate arithmetic opcodes by executing a "similar" (same operation,
different operands) on the cpu.  This ensures accurate emulation, esp. wrt.
eflags.  However, the prologue and epilogue around the opcode is fairly long,
consisting of a switch (for the operand size) and code to load and save the
operands.  This is repeated for every opcode.

This patch introduces an alternative way to emulate arithmetic opcodes.
Instead of the above, we have four (three on i386) functions consisting
of just the opcode and a ret; one for each operand size.  For example:

   .align 8
   em_notb:
	not %al
	ret

   .align 8
   em_notw:
	not %ax
	ret

   .align 8
   em_notl:
	not %eax
	ret

   .align 8
   em_notq:
	not %rax
	ret

The prologue and epilogue are shared across all opcodes.  Note the functions
use a special calling convention; notably eflags is an input/output parameter
and is not clobbered.  Rather than dispatching the four functions through a
jump table, the functions are declared as a constant size (8) so their address
can be calculated.
Acked-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Avi Kivity <avi.kivity@gmail.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>