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

[Jessica Clarke via cfe-commits]

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+# Build clang and compiler-rt for Baremetal RISC-V (cross-compiling)
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jrtc27 wrote:

Well, compiler-rt isn't a compiler, it doesn't target anything

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