cpu/Makefile.include.llvm: Use libstdc++ headers for C++ support

cpu/Makefile.include.llvm

@@ -17,7 +17,39 @@ export DBG         = $(GDBPREFIX)gdb
 # LLVM lacks a binutils strip tool as well...
 #export STRIP      = $(LLVMPREFIX)strip
 
+# Clang on Linux uses GCC's C++ headers and libstdc++ (installed with GCC)
+# Ubuntu and Debian use /etc/alternatives/gcc-$(TARGET_ARCH)-include/c++/$(GCC_VERSION)
+# Arch uses /usr/$(TARGET_ARCH)/include/c++/$(GCC_VERSION)
+# Gentoo uses /usr/lib/gcc/$(TARGET_ARCH)/$(GCC_VERSION)/include/g++-v5
+GCC_CXX_INCLUDE_PATTERNS ?= \
+  /etc/alternatives/gcc-$(TARGET_ARCH)-include/c++/*/ \
+  /usr/$(TARGET_ARCH)/include/c++/*/ \
+  /usr/lib/gcc/$(TARGET_ARCH)/*/include/g++-v5 \
+  #
+
+# Try to find the proper multilib directory using GCC, this may fail if a cross-
+# GCC is not installed.
+ifeq ($(GCC_MULTI_DIR),)
+  GCC_MULTI_DIR := $(shell $(PREFIX)gcc -print-multi-directory $(CFLAGS) 2>/dev/null)
+endif
+
 # Tell clang to cross compile
 export CFLAGS     += -target $(TARGET_ARCH)
 export CXXFLAGS   += -target $(TARGET_ARCH)
 export LINKFLAGS  += -target $(TARGET_ARCH)
+
+# Use the wildcard Makefile function to search for existing directories matching
+# the patterns above. We use the -isystem gcc/clang argument to add the include
+# directories as system include directories, which means they will not be
+# searched until after all the project specific include directories (-I/path)
+# We sort the list of found directories and take the last one, it will likely be
+# the most recent GCC version. This avoids using old headers left over from
+# previous tool chain installations.
+GCC_CXX_INCLUDES ?= \
+    $(addprefix \
+        -isystem $(lastword $(sort \
+            $(foreach pat, $(GCC_CXX_INCLUDE_PATTERNS), $(wildcard $(pat))))), \
+        /. /$(TARGET_ARCH)/$(GCC_MULTI_DIR) /backward \
+    )
+
+export INCLUDES += $(GCC_CXX_INCLUDES)