Rename wasm32-wasi-threads target to wasm32-wasi-preview1-threads

Threading support is based on the [wasi-threads](https://github.com/WebAssembly/wasi-threads/) proposal. It was agreed that the proposal will not be included in any of the WASI releases; instead, there's [another proposal](https://github.com/abrown/thread-spawn) which adds spawning threads functionality to the WASM core.
`wasm32-wasi-threads` is quite a nice name for the long-term solution, therefore I think it should be reserved for the `thread-spawn` proposal and use it once the proposal is ready. Given we'd like to support both `wasi-threads` and `thread-spawn` (at least for some time), I'm proposing adding another target specifically for wasi threads.

I used the `wasm32-wasi-preview1-threads` target as the proposal kind of builds on top of wsp1. It's also the same as the [Rust target](https://github.com/rust-lang/rust/blob/master/src/doc/rustc/src/platform-support/wasm32-wasi-preview1-threads.md). I don't have strong opininion about the name and I'm open for suggestions; if there's a better name for that target, I'd be happy to update the PR.

I keep both `wasm32-wasi-threads` and `wasm32-wasi-preview1-threads` for backward/forward compatibility. I know that `wasm32-wasi-threads` was considered experimental and would probably be safe to delete, but at the same time I think keeping it around is very little effort and can be re-used for thread-spawn in the future. If anybody thinks it's confusing and not worth it, happy to remove it.

I also updated the `thread-spawn` import to be consistent with other wsp1 imports.

.github/workflows/main.yml

@@ -110,7 +110,15 @@ jobs:
         name: ${{ format( 'sysroot-{0}.tgz', matrix.os) }}
         path: sysroot
 
-    - name: Build libc + threads
+    - name: Build libc + threads (wasm32-wasi-preview1-threads)
+      # Only build the thread-capable wasi-libc in the latest supported Clang
+      # version; the earliest version does not have all necessary builtins
+      # (e.g., `__builtin_wasm_memory_atomic_notify`).
+      if: matrix.clang_version != '10.0.0'
+      shell: bash
+      run: make -j4 THREAD_MODEL=posix THREAD_IMPL=wasi-threads
+
+    - name: Build libc + threads (wasm32-wasi-threads)
       # Only build the thread-capable wasi-libc in the latest supported Clang
       # version; the earliest version does not have all necessary builtins
       # (e.g., `__builtin_wasm_memory_atomic_notify`).

Makefile

@@ -38,8 +38,15 @@ TARGET_TRIPLE = wasm32-wasi
 # Threaded version necessitates a different traget, as objects from different
 # targets can't be mixed together while linking.
 ifeq ($(THREAD_MODEL), posix)
+ifeq ($(THREAD_IMPL), wasi-threads)
+TARGET_TRIPLE = wasm32-wasi-preview1-threads
+else
+# The target tripple is deprecated and only kept for backward compatibility.
+# This target will most likely be re-used for implementing thread-spawn proposal
+# in the future.
 TARGET_TRIPLE = wasm32-wasi-threads
 endif
+endif
 
 # These variables describe the locations of various files and directories in
 # the source tree.

README.md

@@ -50,8 +50,8 @@ cross-compiled builds of compiler-rt, `libc++.a`, and `libc++abi.a`.
 
 To enable pthreads support via the [wasi-threads] proposal, follow the above
 build directions with one addition: `make ... THREAD_MODEL=posix`. This creates
-additional artifacts in `sysroot/lib/wasm32-wasi-threads` to support `--target
-wasm32-wasi-threads`.
+additional artifacts in `sysroot/lib/wasm32-wasi-preview1-threads` to support `--target
+wasm32-wasi-preview1-threads`.
 
 ## Arch Linux AUR package