[llvm] [WebAssembly] Implement %llvm.thread.pointer intrinsic. NFC (PR #117817)

[Derek Schuff via llvm-commits]

dschuff wrote:

Generally LGTM; maybe we also want to add a test for `__builtin_thread_pointer` to clang/test/CodeGen/builtins-wasm.c

https://github.com/llvm/llvm-project/pull/117817