Add newlib toolchain with llvm build flow

Update readme about newlib toolchain flow

Co-authored-by: Yun Hsiang <yun.hsiang@sifive.com>

Makefile.in

@@ -938,12 +938,26 @@ stamps/build-llvm-linux: $(LLVM_SRCDIR) $(LLVM_SRC_GIT) \
 	$(MAKE) -C $(notdir $@)
 	$(MAKE) -C $(notdir $@) install
 	cp $(notdir $@)/lib/riscv$(XLEN)-unknown-linux-gnu/libc++* $(SYSROOT)/lib
-	cd $(INSTALL_DIR)/bin && ln -s -f clang $(LINUX_TUPLE)-clang
+	cd $(INSTALL_DIR)/bin && ln -s -f clang $(LINUX_TUPLE)-clang && ln -s -f clang++ $(LINUX_TUPLE)-clang++
 	mkdir -p $(dir $@) && touch $@
 
-stamps/build-llvm-newlib:
-	echo "Building LLVM is only supported in combination with a Linux toolchain."
-	exit 1
+stamps/build-llvm-newlib: $(LLVM_SRCDIR) $(LLVM_SRC_GIT) stamps/build-gcc-newlib-stage2
+	rm -rf $@ $(notdir $@)
+	mkdir $(notdir $@)
+	cd $(notdir $@) && \
+	    cmake $(LLVM_SRCDIR)/llvm \
+	    -DCMAKE_INSTALL_PREFIX=$(INSTALL_DIR) \
+	    -DCMAKE_BUILD_TYPE=Release \
+	    -DLLVM_TARGETS_TO_BUILD="RISCV" \
+	    -DLLVM_ENABLE_PROJECTS="clang;lld" \
+	    -DLLVM_DEFAULT_TARGET_TRIPLE="$(NEWLIB_TUPLE)" \
+	    -DLLVM_INSTALL_TOOLCHAIN_ONLY=On \
+	    -DLLVM_PARALLEL_LINK_JOBS=4
+	$(MAKE) -C $(notdir $@)
+	$(MAKE) -C $(notdir $@) install
+	cd $(INSTALL_DIR)/bin && ln -s -f clang $(NEWLIB_TUPLE)-clang && \
+	    ln -s -f clang++ $(NEWLIB_TUPLE)-clang++
+	mkdir -p $(dir $@) && touch $@
 
 stamps/build-dejagnu: $(DEJAGNU_SRCDIR) $(DEJAGNU_SRC_GIT)
 	rm -rf $@ $(notdir $@)

README.md

@@ -260,12 +260,12 @@ can be used:
 
 Note, that a combination of `--enable-llvm` and multilib configuration flags
 is not supported.
-Also note, that building LLVM is only supported in combination with building
-a Linux toolchain.
 
-Below is an example how to build a rv64gc Linux toolchain with LLVM support,
+Below are examples how to build a rv64gc Linux/newlib toolchain with LLVM support,
 how to use it to build a C and a C++ application using clang, and how to
-execute the generated binaries using QEMU:
+execute the generated binaries using QEMU.
+
+Build Linux toolchain and run examples:
 
     # Build rv64gc toolchain with LLVM
     ./configure --prefix=$RISCV --enable-llvm --enable-linux --with-arch=rv64gc --with-abi=lp64d
@@ -277,6 +277,18 @@ execute the generated binaries using QEMU:
     $RISCV/bin/clang++ -march=rv64imafdc -stdlib=libc++ -o hello_world_cpp hello_world_cpp.cxx
     $RISCV/bin/qemu-riscv64 -L $RISCV/sysroot ./hello_world_cpp
 
+Build newlib toolchain and run examples (don't work with `--with-multilib-generator=`):
+
+    # Build rv64gc bare-metal toolchain with LLVM
+    ./configure --prefix=$RISCV --enable-llvm --disable-linux --with-arch=rv64gc --with-abi=lp64d
+    make -j$(nproc) all build-sim SIM=qemu
+    # Build C application with clang
+    $RISCV/bin/clang -march=rv64imafdc -o hello_world hello_world.c
+    $RISCV/bin/qemu-riscv64 -L $RISCV/sysroot ./hello_world
+    # Build C++ application with clang using static link
+    $RISCV/bin/clang++ -march=rv64imafdc -static -o hello_world_cpp hello_world_cpp.cxx
+    $RISCV/bin/qemu-riscv64 -L $RISCV/sysroot ./hello_world_cpp
+
 ### Development
 
 This section is only for developer or advanced user, or you want to build