target/i386/emulate: mostly rewrite flags handling

While Bochs's algorithms are pretty solid, there are small opportunities
to improve them or to make their logic more similar to TCG's handling
of condition codes.

- use a single bit for the difference between bits 0..7 of result and PF.
This is useful because "set only ZF" is not a common case.

- place SD in the same place as SF

- move CF and PO at bits 62 and 63 when target_ulong is 64-bits wide,
  so that 64-bit ALU operations need fewer shifts

- use rotates to move CF and AF from auxbits to their eflags position

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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