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
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