After snapshot, update the abi constants in syntax/abi.rs

[nikomatsakis]

In syntax/abi.rs (not yet merged in), I had to hardcode various constants due to limitations in the stage0 compiler that appear to have been fixed by others in the meantime.

[pnkfelix]

I'm insufficiently familiar with the release process to judge how such a change should land. So @nikomatsakis I'm going to assign this to you; if you unassign it from yourself, please also de-milestone it.

[brson]

Is this necessary for 0.6?

[nikomatsakis]

Just code cleanup. Demilestone.