[clang] [llvm] [cmake] Add config for riscv(32|64)-unknown-elf (PR #176353)

[Ramkumar Ramachandra via cfe-commits]

artagnon wrote:

Okay, so the problem was macOS: compiler-rt CMake is broken for Darwin in many ways. The hacks have been reverted with a note. Note that libcxx and libcxxabi cannot be built on baremetal until https://github.com/llvm/llvm-project/pull/176346 is finished.

https://github.com/llvm/llvm-project/pull/176353