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

[Petr Hosek via cfe-commits]

petrhosek wrote:

https://github.com/llvm/llvm-project/blob/f53c2e69d919ee3b44dd30f7a75ee25d55eab8a9/clang/cmake/caches/Pico.cmake is another example of a cache file that be used to build a baremetal toolchain that supports Arm and RISC-V, which can be used to build binaries for RP2040 and RP2350.

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