Change wasm32-wasi-pthread to wasm32-wasi-threads

After some thought, I think that we should rename the `THREAD_MODEL=posix` build to avoid confusion. Why? Though in this project the use of this target does involve pthreads, it will not be so in other standard libraries or languages (see, e.g., rust-lang/compiler-team#574). I think it would be preferable to emphasize the "threads" Wasm-level proposal and the "wasi-threads" proposal rather than the specific details of which threading API is being exposed.

Makefile

@@ -38,7 +38,7 @@ 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)
-TARGET_TRIPLE = wasm32-wasi-pthread
+TARGET_TRIPLE = wasm32-wasi-threads
 endif
 
 # These variables describe the locations of various files and directories in