configure runtime ABI by platform

[graydon]

The current runtime ABI is pretty x86-specific (including hard-wired number of
callee-saves and such). Abstract this out somewhat.

[graydon]

Eventually it'd be nice to push all (or most) of the platform-specifics out of rt entirely.

[graydon]

This is no longer enough of a real issue (we're already multiarch) to warrant a bug. Closing.