XCore target: Make handling of large frames not dependent upon an FP.

eliminateFrameIndex() has been reworked to handle both small & large frames
with either a FP or SP.
An additional Slot is required for Scavenging spills when not using FP for large frames.
Reworked the handling of Register Scavenging.

Whether we are using an FP or not, whether it is a large frame or not,
and whether we are using a large code model or not are now independent.