[clang] [llvm] [RISCV] Add big-endian support to RISC-V backend (PR #146534)

[Sam Elliott via cfe-commits]

lenary wrote:

What is your intention here around the LLVM 21 release cycle, knowing this is a big feature and we are 1 week from the branch date

https://github.com/llvm/llvm-project/pull/146534