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

[Ramkumar Ramachandra via cfe-commits]

artagnon wrote:

> > > > the inclusion of LLVMExports relies on the target system supporting shared libraries
> > > 
> > > 
> > > But you're building the compiler for the host, not the target, so why does that matter?
> > 
> > 
> > Yes, the issue is that I get CMake errors when building compiler-rt (in its cmake:112, we call load_llvm_config()), because I set the SYSTEM_NAME while building it to Generic?
> 
> Building for baremetal targets has been done for many years without such issue, why is this one special?

I was also confused when it didn't work out-of-the-box, and maybe I'm still doing something wrong.

> And load_llvm_config is gated on NOT LLVM_RUNTIMES_BUILD...

This doesn't match when the (BUILTINS|RUNTIMES)_$TARGET_FOO way of building baremetal builtins/runtimes is used, unfortunately. I think it's relatively new, and probably doesn't work without our patch? Maybe we can patch load_llvm_config instead? I'm not sure how though?

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