[clang] [llvm] [WIP][RISCV] Support __builtin_cpu_init and __builtin_cpu_supports (PR #99700)

[Piyou Chen via llvm-commits]

BeMg wrote:

> * Updating clang tests to exercise this logic.
> * Actually running it at all.  I did not build compiler-rt, and thus all my checking was of generated asm/IR.

Maybe we could compile the test code with `compiler-rt/lib/builtins/riscv/feature_bits.c` manually. Then we could get rid of copmiler-rt building during running the `__builtin_cpu_init` and `__builtin_cpu_supports` actually. 

Actually, I tried this patch in my local, and it work well. 

---

Here is build script for compiler-rt, if you want to build compiler-rt.

```
cmake -DCMAKE_C_COMPILER_TARGET="riscv64-unknown-linux-gnu" \
      -DCMAKE_ASM_COMPILER_TARGET="riscv64-unknown-linux-gnu" \
      -DCMAKE_AR=./path/to/llvm/install/bin/llvm-ar \
      -DCMAKE_C_COMPILER=./path/to/llvm/install/bin/clang \
      -DCMAKE_NM=./path/to/llvm/install/bin/llvm-nm \
      -DCMAKE_RANLIB=./path/to/llvm/install/bin/llvm-ranlib \
      -DCOMPILER_RT_BUILD_BUILTINS=ON \
      -DCOMPILER_RT_BUILD_LIBFUZZER=OFF \
      -DCOMPILER_RT_BUILD_MEMPROF=OFF \
      -DCOMPILER_RT_BUILD_PROFILE=OFF \
      -DCOMPILER_RT_BUILD_SANITIZERS=OFF \
      -DCOMPILER_RT_BUILD_XRAY=OFF \
      -DCOMPILER_RT_DEFAULT_TARGET_ONLY=ON \
      -DLLVM_CONFIG_PATH=./path/to/llvm/install/bin/llvm-config \
      -G "Ninja" ../compiler-rt
ninja 
# Put the `libclang_rt.builtins-riscv64.a` in correct path
cp ./path/to/build/lib/linux/* ./path/to/llvm/install/lib/clang/19/lib/linux
```

```
# Compile using compiler-rt
clang -march=rv64imafd -rtlib=compiler-rt test.c
```

> * Investigate claims from gcc docs that __builtin_cpu_init is called early in process lifetime with high priority constructor.  I did not find this with some quick searching.

https://github.com/llvm/llvm-project/pull/99700