Modularize handling of x64 ABI rules in native.rs

[graydon]

Currently native.rs has a lot of alt switching on whether or not we're generating calls to x64-ABI or not. This should be factored into arch-specific and arch-neutral pieces, with x64 being only one such arch.

[pnkfelix]

Not critical for 0.6; de-milestoning

[sanxiyn]

@crabtw, didn't #4590 fix this?

[crabtw]

@sanxiyn yes, that should fix this issue.

[pcwalton]

Fixed.