x86-64: correct / adjust prefix emission

x86-64: correct / adjust prefix emission

First and foremost REX must come last. Next JumpInterSegment branches
can't possibly have a REX prefix, as they're consistently CpuNo64. And
finally make BND prefix handling in output_branch() consistent with that
of other prefixes in the same function, and make its placement among
prefixes consistent with output_jump() (which, oddly enough, still isn't
the supposedly canonical order specified by the *_PREFIX definitions).