x86_emulate: Emit emulated forms of most FPU instructions as '.byte

x86_emulate: Emit emulated forms of most FPU instructions as '.byte
xx,yy'. This is arguably clearer than using the mnemonic opcode, since
it is more clearly the instruction we have just decoded. Furthermore,
gas likes to reverse FPU operands on some two-operand FPU instructions
for historical reasons. Finally, 'byte xx,yy' is potentially more
amenable to further macro-isation down the road.
Signed-off-by: Keir Fraser <keir.fraser@citrix.com>