Implements foundation for RISC-V 64-bit (riscv64) target support with RISC-V
Vector Extension (RVV) ISA, introducing the rvv-x4 target for 4-wide
vectorization.
Key additions:
- RISCV_ENABLED CMake configuration option
- RISC-V target enumeration and ISA definitions
- LLVM backend initialization for RISC-V
- RISC-V target with 4-wide vectorization
- Architecture detection and triple configuration