test: add tests for pthread API

This change runs the pthread tests available in `libc-test` when
`THREAD_MODEL=posix`. This is currently a work-in-progress, since there
are custom build steps necessary to get this all to (mostly) work.
Currently this might look like:

```console
$ cd test
$ make run THREAD_MODEL=posix \
    ENGINE=~/Code/wasmtime/target/debug/wasmtime
    CC=~/Code/llvm-project/build/bin/clang
```

What are the current issues?
- this requires a special build of Wasmtime that includes the
  `wasi-threads` implementation; this is shown above as built from
  source
- under `THREAD_MODEL=posix`, this requires Clang to understand the
  `--export-memory` flag which, though merged in
  https://reviews.llvm.org/D135898, is not available for download yet
  (but can be built from latest source as shown above)
- not all of the tests yet pass and some do not build: this is to be
  expected in several cases (PI? robust?) but there is perhaps more that
  can be done here to enable more of the pthread API. Only the working
  tests are included in this commit.

test/Makefile

@@ -15,6 +15,10 @@ test: run
 # directory (keeping with the `wasi-libc` conventions).
 OBJDIR ?= $(CURDIR)/build
 DOWNDIR ?= $(CURDIR)/download
+# Like the top-level `wasi-libc` Makefile, here we can decide whether to test
+# with threads (`posix`) or without (`single`). IMPORTANT: the top-level include
+# directory must be built with the same value.
+THREAD_MODEL ?= single
 
 ##### DOWNLOAD #################################################################
 
@@ -90,6 +94,14 @@ TESTS := \
 	$(LIBC_TEST)/src/functional/wcsstr.c \
 	$(LIBC_TEST)/src/functional/wcstol.c
 
+ifeq ($(THREAD_MODEL), posix)
+	TESTS += \
+		$(LIBC_TEST)/src/functional/pthread_cond.c \
+		$(LIBC_TEST)/src/functional/pthread_tsd.c \
+		$(LIBC_TEST)/src/functional/tls_init.c \
+		$(LIBC_TEST)/src/functional/tls_local_exec.c
+endif
+
 # Part of the problem including more tests is that the `libc-test`
 # infrastructure code is not all Wasm-compilable. As we include more tests
 # above, this list will also likely need to grow.
@@ -116,23 +128,54 @@ ifeq ($(origin CC), default)
 CC := clang
 endif
 LDFLAGS ?=
-CFLAGS ?= --target=wasm32-wasi --sysroot=../sysroot
+CFLAGS ?= --sysroot=../sysroot
 # Always include the `libc-test` infrastructure headers.
 CFLAGS += -I$(LIBC_TEST)/src/common
 
+# Configure support for threads.
+ifeq ($(THREAD_MODEL), single)
+CFLAGS += --target=wasm32-wasi -mthread-model single
+endif
+ifeq ($(THREAD_MODEL), posix)
+# Specify the tls-model until LLVM 15 is released (which should contain
+# https://reviews.llvm.org/D130053).
+CFLAGS += --target=wasm32-wasi-pthread -mthread-model posix -pthread -ftls-model=local-exec
+# NOTE: `--export-memory` is invalid until https://reviews.llvm.org/D135898 is
+# available in a released version of `wasm-ld`; this has not happened yet so
+# a built-from-latest-source Clang is necessary.
+LDFLAGS += -Wl,--import-memory,--export-memory,--max-memory=1048576
+endif
+
+# We want to be careful to rebuild the Wasm files if any important variables
+# change. This block calculates a hash from key variables and writes a file
+# (ENV_HASH = `build/env-<hash>`); any targets dependent on `ENV_HASH` will thus
+# be rebuilt whenever `ENV_HASH` is modified, which is whenever `ENV` changes.
+# The phony `env` target can be used to debug this.
+ENV = "CC=$(CC);CFLAGS=$(CFLAGS);LDFLAGS=$(LDFLAGS);THREAD_MODEL=$(THREAD_MODEL)"
+export ENV
+ENV_HASH = $(OBJDIR)/env-$(shell echo $(ENV) | md5sum | cut -d ' ' -f 1)
+$(ENV_HASH): | $(OBJDIRS)
+	rm -f $(OBJDIR)/env-*
+	echo $(ENV) > $@
+env: $(ENV_HASH)
+	@echo ENV = "$$ENV"
+	@echo ENV_HASH = $(ENV_HASH)
+	cat $^
+.PHONY: env
+
 # Compile each selected test using Clang. Note that failures here are likely
 # due to a missing `libclang_rt.builtins-wasm32.a` in the Clang lib directory.
 # This location is system-dependent, but could be fixed by something like:
 #  $ sudo mkdir /usr/lib64/clang/14.0.5/lib/wasi
 #  $ sudo cp download/lib/wasi/libclang_rt.builtins-wasm32.a /usr/lib64/clang/14.0.5/lib/wasi/
-build: download $(WASMS)
+build: download $(WASMS) $(ENV_HASH)
 
 $(WASMS): | $(OBJDIRS)
-$(OBJDIR)/%.wasm: $(OBJDIR)/%.wasm.o $(INFRA_WASM_OBJS)
-	$(CC) $(CFLAGS) $(LDFLAGS) -o $@ $^ 
+$(OBJDIR)/%.wasm: $(OBJDIR)/%.wasm.o $(INFRA_WASM_OBJS) $(ENV_HASH)
+	$(CC) $(CFLAGS) $(LDFLAGS) -o $@ $(filter-out $(ENV_HASH), $^) 
 
 $(WASM_OBJS): $(LIBC_TEST)/src/common/test.h | $(OBJDIRS)
-$(OBJDIR)/%.wasm.o: $(LIBC_TEST)/src/%.c
+$(OBJDIR)/%.wasm.o: $(LIBC_TEST)/src/%.c $(ENV_HASH)
 	$(CC) $(CFLAGS) -c -o $@ $< 
 
 $(OBJDIRS):
@@ -144,6 +187,7 @@ clean::
 ##### RUN ######################################################################
 
 ENGINE ?= $(WASMTIME) run
+ENGINE_FLAGS ?= --wasm-features threads --wasi-modules experimental-wasi-threads
 ERRS:=$(WASMS:%.wasm=%.wasm.err)
 
 # Use the provided Wasm engine to execute each test, emitting its output into
@@ -154,7 +198,7 @@ run: build $(ERRS)
 $(ERRS): | $(OBJDIRS)
 
 %.wasm.err: %.wasm
-	$(ENGINE) $< >$@
+	$(ENGINE) $(ENGINE_FLAGS) $<
 
 clean::
 	rm -rf $(OBJDIR)/*/*.err