Add the ARC backend to lib/Target.
- Targets ARC v2.
- Includes a small functional subset of the 32-bit ISA, little endian.
- Targets a fixed subset of the ARC v2 ISA.
- Focuses on C99 support.
- Implements Target registration, and a SelectionDAG based instruction selector.
- Implements Register and the ISA specification for current subset.
- Implements the Assembly printer and disassembler.
I think will be good idea to make namespace formatting uniform across files. It looks better when empty line separate namespace beginning and end. end namspace XYZ and end anonymous namespace are predominant closing comments in LLVM.