[RISCV] Add initial assembler/MC layer support for big-endian (#146534)

This patch adds basic assembler and MC layer infrastructure for
RISC-V big-endian targets (riscv32be/riscv64be):
      - Register big-endian targets in RISCVTargetMachine
      - Add big-endian data layout strings
      - Implement endianness-aware fixup application in assembler
        backend
      - Add byte swapping for data fixups on BE cores
      - Update MC layer components (AsmInfo, MCTargetDesc, Disassembler,
        AsmParser)
    
This provides the foundation for BE support but does not yet include:
      - Codegen patterns for BE
      - Load/store instruction handling
      - BE-specific subtarget features

0 files changed, 0 insertions, 0 deletions