[AMDGPU] Rename Bonaire target to be gfx704; update target feature handling

[AMDGPU] Rename Bonaire target to be gfx704; update target feature handling

- Rename Bonaire target to be gfx704.
- Eliminate gfx800 and make Iceland and Tonga both use gfx802 as they use the same code.
- List target features supported by each processor in the processor table together with the default value.
- Add xnack flag to e_flags.
- Remove xnack from kernel metadata and kernel descriptor since it is now a whole code object property.

Differential Revision: https://reviews.llvm.org/D40051

llvm-svn: 320378