Add experimental wasi-threads build

eg.
```
autoconf
export WASI_SDK_PATH=/opt/wasi-sdk-19.5g0236e959edbc
./Tools/wasm/wasm_build.py wasi-threads build
~/git/toywasm/b.thread/toywasm --wasi --wasi-dir . --wasi-dir ~/git/garbage/py/th2 -- builddir/wasi-threads/python.wasm ~/git/garbage/py/th2/thread.py
```

* I used an unreleased version of wasi-sdk

* I used toywasm to test because wasmtime doesn't have
  necessary functionality yet.

Python/thread_pthread.h

@@ -356,7 +356,14 @@ PyThread_exit_thread(void)
 {
     if (!initialized)
         exit(0);
+#if defined(__wasi__)
+    /*
+     * wasi-threads doesn't have pthread_exit right now
+     */
+    abort();
+#else
     pthread_exit(0);
+#endif
 }
 
 #ifdef USE_SEMAPHORES

Tools/wasm/config.site-wasm32-wasi

@@ -40,3 +40,5 @@ ac_cv_header_netpacket_packet_h=no
 
 # Disable int-conversion for wask-sdk as it triggers an error from version 17.
 ac_cv_disable_int_conversion=yes
+
+ac_cv_pthread=yes

Tools/wasm/wasm_build.py

@@ -480,7 +480,6 @@ def configure_cmd(self) -> List[str]:
             cmd.append(f"--{opt}-wasm-dynamic-linking")
 
         if self.pthreads is not None:
-            assert self.host.is_emscripten
             opt = "enable" if self.pthreads else "disable"
             cmd.append(f"--{opt}-wasm-pthreads")
 
@@ -745,6 +744,13 @@ def build_emports(self, force: bool = False):
         support_level=SupportLevel.supported,
         host=Host.wasm32_wasi,
     ),
+    # wasm32-wasi-threads
+    BuildProfile(
+        "wasi-threads",
+        support_level=SupportLevel.experimental,
+        host=Host.wasm32_wasi,
+        pthreads=True,
+    ),
     # no SDK available yet
     # BuildProfile(
     #    "wasm64-wasi",

configure.ac

@@ -1053,7 +1053,11 @@ cat > conftest.c <<EOF
 #  if defined(__EMSCRIPTEN__)
 	wasm32-emscripten
 #  elif defined(__wasi__)
+#    if defined(_REENTRANT)
+	wasm32-wasi-threads
+#    else
 	wasm32-wasi
+#    endif
 #  else
 #    error unknown wasm32 platform
 #  endif
@@ -1245,7 +1249,7 @@ AC_ARG_ENABLE([wasm-pthreads],
 [
   AS_CASE([$ac_sys_system],
     [Emscripten], [],
-    [WASI], [AC_MSG_ERROR([WASI threading is not implemented yet.])],
+    [WASI], [],
     [AC_MSG_ERROR([--enable-wasm-pthreads only applies to Emscripten and WASI])]
   )
 ], [
@@ -2199,6 +2203,15 @@ AS_CASE([$ac_sys_system],
     LIBS="$LIBS -lwasi-emulated-signal -lwasi-emulated-getpid -lwasi-emulated-process-clocks"
     echo "#define _WASI_EMULATED_SIGNAL 1" >> confdefs.h
 
+    AS_VAR_IF([enable_wasm_pthreads], [yes], [
+      # Note: update CFLAGS because ac_compile/ac_link needs this too
+      CFLAGS="$CFLAGS -target wasm32-wasi-threads -pthread"
+      AS_VAR_APPEND([CFLAGS_NODIST], [" -target wasm32-wasi-threads -pthread"])
+      AS_VAR_APPEND([LDFLAGS_NODIST], [" -target wasm32-wasi-threads -pthread"])
+      AS_VAR_APPEND([LDFLAGS_NODIST], [" -Wl,--import-memory"])
+      AS_VAR_APPEND([LDFLAGS_NODIST], [" -Wl,--max-memory=10485760"])
+    ])
+
     dnl increase initial memory and stack size, move stack first
     dnl https://github.com/WebAssembly/wasi-libc/issues/233
     AS_VAR_APPEND([LDFLAGS_NODIST], [" -z stack-size=524288 -Wl,--stack-first -Wl,--initial-memory=10485760"])