add shared library support (#338)

This adds support for building WASI shared libraries per
https://github.com/WebAssembly/tool-conventions/blob/main/DynamicLinking.md.

For the time being, the goal is to allow "pseudo-dynamic" linking using the
Component Model per
https://github.com/WebAssembly/component-model/blob/main/design/mvp/examples/SharedEverythingDynamicLinking.md.
This requires all libraries to be available when the component is created, but
still allows runtime symbol resolution via `dlopen`/`dlsym` backed by a static
lookup table.  This is sufficient to support Python native extensions, for
example.  A complete demo using `wit-component` is available at
https://github.com/dicej/component-linking-demo.

Signed-off-by: Joel Dice <joel.dice@fermyon.com>

.gitmodules

@@ -3,7 +3,7 @@
 	url = https://github.com/llvm/llvm-project
 [submodule "src/wasi-libc"]
 	path = src/wasi-libc
-	url = https://github.com/CraneStation/wasi-libc
+	url = https://github.com/WebAssembly/wasi-libc
 [submodule "src/config"]
 	path = src/config
 	url = https://git.savannah.gnu.org/git/config.git

Makefile

@@ -107,12 +107,13 @@ build/llvm.BUILT:
 		llvm-config
 	touch build/llvm.BUILT
 
-build/wasi-libc.BUILT: build/llvm.BUILT
+build/wasi-libc.BUILT: build/compiler-rt.BUILT
 	$(MAKE) -C $(ROOT_DIR)/src/wasi-libc \
 		CC=$(BUILD_PREFIX)/bin/clang \
 		AR=$(BUILD_PREFIX)/bin/llvm-ar \
 		NM=$(BUILD_PREFIX)/bin/llvm-nm \
-		SYSROOT=$(BUILD_PREFIX)/share/wasi-sysroot
+		SYSROOT=$(BUILD_PREFIX)/share/wasi-sysroot \
+		default libc_so
 	$(MAKE) -C $(ROOT_DIR)/src/wasi-libc \
 		CC=$(BUILD_PREFIX)/bin/clang \
 		AR=$(BUILD_PREFIX)/bin/llvm-ar \
@@ -121,7 +122,7 @@ build/wasi-libc.BUILT: build/llvm.BUILT
 		THREAD_MODEL=posix
 	touch build/wasi-libc.BUILT
 
-build/compiler-rt.BUILT: build/llvm.BUILT build/wasi-libc.BUILT
+build/compiler-rt.BUILT: build/llvm.BUILT
 	# Do the build, and install it.
 	mkdir -p build/compiler-rt
 	cd build/compiler-rt && cmake -G Ninja \
@@ -151,48 +152,52 @@ build/compiler-rt.BUILT: build/llvm.BUILT build/wasi-libc.BUILT
 	touch build/compiler-rt.BUILT
 
 # Flags for libcxx and libcxxabi.
+# $(1): pthreads ON or OFF
+# $(2): shared libraries ON or OFF
 LIBCXX_CMAKE_FLAGS = \
     -DCMAKE_C_COMPILER_WORKS=ON \
     -DCMAKE_CXX_COMPILER_WORKS=ON \
     -DCMAKE_AR=$(BUILD_PREFIX)/bin/ar \
     -DCMAKE_MODULE_PATH=$(ROOT_DIR)/cmake \
     -DCMAKE_TOOLCHAIN_FILE=$(ROOT_DIR)/wasi-sdk.cmake \
     -DCMAKE_STAGING_PREFIX=$(PREFIX)/share/wasi-sysroot \
+    -DCMAKE_POSITION_INDEPENDENT_CODE=$(2) \
     -DLLVM_CONFIG_PATH=$(ROOT_DIR)/build/llvm/bin/llvm-config \
     -DCMAKE_VERBOSE_MAKEFILE:BOOL=ON \
     -DCXX_SUPPORTS_CXX11=ON \
-    -DLIBCXX_ENABLE_THREADS:BOOL=@PTHREAD@ \
-    -DLIBCXX_HAS_PTHREAD_API:BOOL=@PTHREAD@ \
+    -DLIBCXX_ENABLE_THREADS:BOOL=$(1) \
+    -DLIBCXX_HAS_PTHREAD_API:BOOL=$(1) \
     -DLIBCXX_HAS_EXTERNAL_THREAD_API:BOOL=OFF \
     -DLIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY:BOOL=OFF \
     -DLIBCXX_HAS_WIN32_THREAD_API:BOOL=OFF \
     -DLLVM_COMPILER_CHECKED=ON \
     -DCMAKE_BUILD_TYPE=RelWithDebugInfo \
-    -DLIBCXX_ENABLE_SHARED:BOOL=OFF \
+    -DLIBCXX_ENABLE_SHARED:BOOL=$(2) \
     -DLIBCXX_ENABLE_EXPERIMENTAL_LIBRARY:BOOL=OFF \
     -DLIBCXX_ENABLE_EXCEPTIONS:BOOL=OFF \
     -DLIBCXX_ENABLE_FILESYSTEM:BOOL=OFF \
+    -DLIBCXX_ENABLE_ABI_LINKER_SCRIPT:BOOL=OFF \
     -DLIBCXX_CXX_ABI=libcxxabi \
     -DLIBCXX_CXX_ABI_INCLUDE_PATHS=$(LLVM_PROJ_DIR)/libcxxabi/include \
     -DLIBCXX_HAS_MUSL_LIBC:BOOL=ON \
     -DLIBCXX_ABI_VERSION=2 \
     -DLIBCXXABI_ENABLE_EXCEPTIONS:BOOL=OFF \
-    -DLIBCXXABI_ENABLE_SHARED:BOOL=OFF \
+    -DLIBCXXABI_ENABLE_SHARED:BOOL=$(2) \
     -DLIBCXXABI_SILENT_TERMINATE:BOOL=ON \
-    -DLIBCXXABI_ENABLE_THREADS:BOOL=@PTHREAD@ \
-    -DLIBCXXABI_HAS_PTHREAD_API:BOOL=@PTHREAD@ \
+    -DLIBCXXABI_ENABLE_THREADS:BOOL=$(1) \
+    -DLIBCXXABI_HAS_PTHREAD_API:BOOL=$(1) \
     -DLIBCXXABI_HAS_EXTERNAL_THREAD_API:BOOL=OFF \
     -DLIBCXXABI_BUILD_EXTERNAL_THREAD_LIBRARY:BOOL=OFF \
     -DLIBCXXABI_HAS_WIN32_THREAD_API:BOOL=OFF \
-    -DLIBCXXABI_ENABLE_PIC:BOOL=OFF \
+    -DLIBCXXABI_ENABLE_PIC:BOOL=$(2) \
     -DWASI_SDK_PREFIX=$(BUILD_PREFIX) \
     -DUNIX:BOOL=ON \
     --debug-trycompile
 
-build/libcxx.BUILT: build/llvm.BUILT build/compiler-rt.BUILT build/wasi-libc.BUILT
+build/libcxx.BUILT: build/llvm.BUILT build/wasi-libc.BUILT
 	# Do the build.
 	mkdir -p build/libcxx
-	cd build/libcxx && cmake -G Ninja $(LIBCXX_CMAKE_FLAGS:@PTHREAD@=OFF) \
+	cd build/libcxx && cmake -G Ninja $(call LIBCXX_CMAKE_FLAGS,OFF,ON) \
 		-DCMAKE_SYSROOT=$(BUILD_PREFIX)/share/wasi-sysroot \
 		-DCMAKE_C_FLAGS="$(DEBUG_PREFIX_MAP) $(EXTRA_CFLAGS)" \
 		-DCMAKE_CXX_FLAGS="$(DEBUG_PREFIX_MAP) $(EXTRA_CXXFLAGS)" \
@@ -202,7 +207,7 @@ build/libcxx.BUILT: build/llvm.BUILT build/compiler-rt.BUILT build/wasi-libc.BUI
 		$(LLVM_PROJ_DIR)/runtimes
 	ninja $(NINJA_FLAGS) -C build/libcxx
 	mkdir -p build/libcxx-threads
-	cd build/libcxx-threads && cmake -G Ninja $(LIBCXX_CMAKE_FLAGS:@PTHREAD@=ON) \
+	cd build/libcxx-threads && cmake -G Ninja $(call LIBCXX_CMAKE_FLAGS,ON,OFF) \
 		-DCMAKE_SYSROOT=$(BUILD_PREFIX)/share/wasi-sysroot \
 		-DCMAKE_C_FLAGS="$(DEBUG_PREFIX_MAP) -pthread $(EXTRA_CFLAGS)" \
 		-DCMAKE_CXX_FLAGS="$(DEBUG_PREFIX_MAP) -pthread $(EXTRA_CXXFLAGS)" \