[WebAssembly] ICE when using __builtin_thread_pointer

[kleisauke]

test.c:

void *f0(void)
{
  return __builtin_thread_pointer();
}

$ /emsdk/upstream/bin/clang --target=wasm32 -o test.wasm test.c
fatal error: error in backend: Cannot select: intrinsic %llvm.thread.pointer
clang: error: clang frontend command failed with exit code 70 (use -v to see invocation)
clang version 20.0.0git (https:/github.com/llvm/llvm-project 3a31427224d4fa49d7ef737b21f6027dc4928ecf)
Target: wasm32
Thread model: posix
InstalledDir: /emsdk/upstream/bin
Build config: +assertions
clang: note: diagnostic msg: 
********************

PLEASE ATTACH THE FOLLOWING FILES TO THE BUG REPORT:
Preprocessed source(s) and associated run script(s) are located at:
clang: note: diagnostic msg: /tmp/test-9adbd1.c
clang: note: diagnostic msg: /tmp/test-9adbd1.sh
clang: note: diagnostic msg: 

********************

Context: emscripten-core/emscripten#21548.