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

[Djordje Todorovic via llvm-commits]

djtodoro wrote:

Support for `clang`, `lld` and `lldb` will be added in later PRs. All together support can be found at https://github.com/djtodoro/llvm-project/tree/pr/riscv-be-support.

Basically, this brings into life the https://reviews.llvm.org/D128612. 

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