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.