Build openmp libraries for Clang/LLVM on RV64

For Clang/LLVM, the default OpenMP library is `libomp`.

However, we don't build openmp library, which will cause a linker error:
```
riscv64-unknown-linux-gnu-ld: cannot find -lomp: No such file or directory
clang: error: linker command failed with exit code 1 (use -v to see invocation)
```

So we build `libomp.so/libomp.a` manually.

Makefile.in

@@ -982,6 +982,47 @@ stamps/build-llvm-linux: $(LLVM_SRCDIR) $(LLVM_SRC_GIT) $(BINUTILS_SRCDIR) $(BIN
 	    -DLLVM_PARALLEL_LINK_JOBS=4
 	$(MAKE) -C $(notdir $@)
 	$(MAKE) -C $(notdir $@) install
+	# Build shared/static OpenMP libraries on RV64.
+	if test $(XLEN) -eq 64; then \
+	    mkdir $(notdir $@)/openmp-shared; \
+	    cmake -S$(LLVM_SRCDIR)/openmp \
+	        -B$(notdir $@)/openmp-shared \
+	        -DCMAKE_INSTALL_PREFIX=$(INSTALL_DIR) \
+	        -DCMAKE_C_COMPILER=$(INSTALL_DIR)/bin/clang \
+	        -DCMAKE_CXX_COMPILER=$(INSTALL_DIR)/bin/clang++ \
+	        -DOPENMP_ENABLE_LIBOMPTARGET=Off \
+	        -DCMAKE_BUILD_TYPE=Release \
+	        -DLIBOMP_ARCH=riscv64 \
+	        -DLIBOMP_HAVE_WARN_SHARED_TEXTREL_FLAG=On \
+	        -DLIBOMP_HAVE_AS_NEEDED_FLAG=On \
+	        -DLIBOMP_HAVE_VERSION_SCRIPT_FLAG=On \
+	        -DLIBOMP_HAVE_STATIC_LIBGCC_FLAG=On \
+	        -DLIBOMP_HAVE_Z_NOEXECSTACK_FLAG=On \
+	        -DDISABLE_OMPD_GDB_PLUGIN=On \
+	        -DLIBOMP_OMPD_GDB_SUPPORT=Off \
+	        -DLIBOMP_ENABLE_SHARED=On; \
+	    $(MAKE) -C $(notdir $@)/openmp-shared; \
+	    $(MAKE) -C $(notdir $@)/openmp-shared install; \
+	    mkdir $(notdir $@)/openmp-static; \
+	    cmake -S$(LLVM_SRCDIR)/openmp \
+	        -B$(notdir $@)/openmp-static \
+	        -DCMAKE_INSTALL_PREFIX=$(INSTALL_DIR) \
+	        -DCMAKE_C_COMPILER=$(INSTALL_DIR)/bin/clang \
+	        -DCMAKE_CXX_COMPILER=$(INSTALL_DIR)/bin/clang++ \
+	        -DOPENMP_ENABLE_LIBOMPTARGET=Off \
+	        -DCMAKE_BUILD_TYPE=Release \
+	        -DLIBOMP_ARCH=riscv64 \
+	        -DLIBOMP_HAVE_WARN_SHARED_TEXTREL_FLAG=On \
+	        -DLIBOMP_HAVE_AS_NEEDED_FLAG=On \
+	        -DLIBOMP_HAVE_VERSION_SCRIPT_FLAG=On \
+	        -DLIBOMP_HAVE_STATIC_LIBGCC_FLAG=On \
+	        -DLIBOMP_HAVE_Z_NOEXECSTACK_FLAG=On \
+	        -DDISABLE_OMPD_GDB_PLUGIN=On \
+	        -DLIBOMP_OMPD_GDB_SUPPORT=Off \
+	        -DLIBOMP_ENABLE_SHARED=Off; \
+	    $(MAKE) -C $(notdir $@)/openmp-static; \
+	    $(MAKE) -C $(notdir $@)/openmp-static install; \
+	fi
 	cp $(notdir $@)/lib/riscv$(XLEN)-unknown-linux-gnu/libc++* $(SYSROOT)/lib
 	cp $(notdir $@)/lib/LLVMgold.so  $(INSTALL_DIR)/lib
 	cd $(INSTALL_DIR)/bin && ln -s -f clang $(LINUX_TUPLE)-clang && ln -s -f clang++ $(LINUX_TUPLE)-clang++