AMDGPU: Refactor processor definition to use ISA version features

Add missing ISA versions 7.0.2/8.0.4/8.1.0. to backend.

Refactor processor definition to use ISA version features.

Fixed ISA version for stoney.

Based on Laurent Morichetti's patch.

Differential Revision: https://reviews.llvm.org/D25919

llvm-svn: 285210