Build compiler-rt for wasm32 and wasm64

Current shortcomings/future work:

 * Unfortunately this currently takes the form of a separate, standalone
   build (with the just-built stage2 toolchain), which makes this a
   little more convoluted. The upstream compiler-rt build system does
   not have a way (that I could find) to manually add additional targets
   to the build.

 * GNU strip (binutils) does not understand the WebAssembly binary, so
   dh_strip fails. llvm-strip does, but a) does not understand all the
   arguments that dh_strip passes (e.g. -N), b) fails with "invalid
   relocation offset" on --strip-debug (but works with --strip-all).

   This currently passes -X to dh_strip to exclude them from being
   stripped entirely. Invoking llvm-strip manually with --strip-all can
   be considered in the future.

* The wasm32/wasm64 binaries are, by design, architecture-independent.
  With this commit, they are being built in every architecture, and
  shipped in the libclang-common-X.Y-dev package, which is arch: any,
  but ships both architecture-dependent (native/host compiler-rt) and
  architecture-independent files (include headers etc.).

  In a future iteration, the wasm32/wasm64 compiler-rt library could be
  factored out to a separate (arch: all) package. This is left
  outstanding because it should probably happen alongside a different
  split for libclang-common-X.Y-dev that will include splitting the
  native libclang-rt as well.

Closes: #1010932

debian/rules

@@ -679,7 +679,38 @@ debian-libclc-build:
 	ninja $(NJOBS) $(VERBOSE)
 	touch $@
 
-override_dh_auto_build: debian-full-build debian-libfuzzer-build debian-libclc-build
+debian-rtlib-wasm-build:
+	echo "Using cmake: $(CMAKE_BIN)"
+	for build in wasm32 wasm64; do \
+		mkdir -p build-compiler-rt-$$build; \
+		$(CMAKE_BIN) -B build-compiler-rt-$$build -S compiler-rt/lib/builtins/ \
+		-G Ninja \
+		-DCMAKE_C_COMPILER_TARGET=$$build-unknown-unknown \
+		-DCMAKE_CXX_COMPILER_TARGET=$$build-unknown-unknown \
+		-DCMAKE_ASM_COMPILER_TARGET=$$build-unknown-unknown \
+		-DCMAKE_C_COMPILER=$(STAGE_2_BIN_DIR)/clang \
+		-DCMAKE_CXX_COMPILER=$(STAGE_2_BIN_DIR)/clang++ \
+		-DCMAKE_C_FLAGS="$(opt_flags) $(STAGE_2_CFLAGS)" \
+		-DCMAKE_CXX_FLAGS="$(opt_flags) $(STAGE_2_CXXFLAGS)" \
+		-DCMAKE_SHARED_LINKER_FLAGS="$(STAGE_2_LDFLAGS) -L$(STAGE_2_LIB_DIR)" \
+		-DCMAKE_MODULE_LINKER_FLAGS="$(STAGE_2_LDFLAGS) -L$(STAGE_2_LIB_DIR)" \
+		-DCMAKE_EXE_LINKER_FLAGS="$(STAGE_2_LDFLAGS) -L$(STAGE_2_LIB_DIR)" \
+		-DCMAKE_INSTALL_PREFIX=/usr/lib/llvm-$(LLVM_VERSION)/lib/clang/$(LLVM_VERSION_FULL) \
+		-DCMAKE_INSTALL_DATADIR=lib \
+		-DCMAKE_INSTALL_INCLUDEDIR=include \
+		-DLLVM_CONFIG_PATH=$(STAGE_2_BIN_DIR)/llvm-config \
+		-DCOMPILER_RT_STANDALONE_BUILD=ON \
+		-DCOMPILER_RT_BAREMETAL_BUILD=ON \
+		-DCOMPILER_RT_INCLUDE_TESTS=OFF \
+		-DCOMPILER_RT_USE_LIBCXX=OFF \
+		-DCOMPILER_RT_USE_BUILTINS_LIBRARY=ON \
+		-DCOMPILER_RT_DEFAULT_TARGET_TRIPLE=$$build-unknown-unknown \
+		-DCOMPILER_RT_OS_DIR=wasi; \
+		ninja -C build-compiler-rt-$$build $(NJOBS) $(VERBOSE); \
+	done
+	touch $@
+
+override_dh_auto_build: debian-full-build debian-libfuzzer-build debian-libclc-build debian-rtlib-wasm-build
 
 override_dh_prep: build_doc
 	dh_prep
@@ -819,6 +850,9 @@ endif
 
 	DESTDIR=$(DEB_INST) ninja $(VERBOSE) -C libclc/build install
 
+	DESTDIR=$(DEB_INST) ninja $(VERBOSE) -C build-compiler-rt-wasm32 install
+	DESTDIR=$(DEB_INST) ninja $(VERBOSE) -C build-compiler-rt-wasm64 install
+
 # Rename binaries
 	mkdir -p $(DEB_INST)/usr/bin/
 	cd $(DEB_INST)/usr/bin/; \
@@ -964,7 +998,9 @@ ifeq ($(shell dpkg --compare-versions $(shell dpkg-query -W -f '$${Version}' bin
 	: # for some reasons, the +x might be removed
 	chmod -f +x $(CURDIR)/debian/*/usr/lib/llvm-$(LLVM_VERSION)/bin/* || true
 else
-	dh_strip -a -v
+	# GNU strip doesn't recognize WebAssembly binaries, and actually corrupts them.
+	# llvm-strip (as of 15.0.2) fails with --strip-debug (but works with --strip-all)
+	dh_strip -a -v -Xlibclang_rt.builtins-wasm32.a -Xlibclang_rt.builtins-wasm64.a
 endif
 
 override_dh_install:
@@ -1098,6 +1134,7 @@ override_dh_auto_clean:
 	rm -f $(CURDIR)/clang/tools/clang-format/clang-format-diff-$(LLVM_VERSION)
 	rm -f $(CURDIR)/clang/tools/clang-format/clang-format-$(LLVM_VERSION).py
 	rm -rf libclc/build
+	rm -rf build-compiler-rt-wasm32 build-compiler-rt-wasm64
 	if test -f lld/docs/ld.lld-$(LLVM_VERSION).1; then \
 		mv lld/docs/ld.lld-$(LLVM_VERSION).1 lld/docs/ld.lld.1; \
 	fi
@@ -1113,4 +1150,4 @@ override_dh_auto_clean:
 	: # remove extra stamps
 	rm -f debian-*-build
 
-.PHONY: override_dh_strip preconfigure debian-full-build debian-libfuzzer-build debian-libclc-build
+.PHONY: override_dh_strip preconfigure debian-full-build debian-libfuzzer-build debian-libclc-build debian-rtlib-wasm-build