RFC: PowerPC floating point features

[David Edelsohn]

	I would prefer feature-based.

	TARGET_HARD_FLOAT represents the presence of FPUs.

	TARGET_FPRS represents the presence of FP register set because
one variant used GPRs for FP operations.

	E500 then added another variant with double-precision FP in the
GPRs. 

	PowerPC is an architecture and the mess of FP variants clearly is
not a architecture allowing compatibility.

David