Makefile.include: Introduce LINKXX variable

This variable can be used to specify the linker which should be used to
link C++ object files. This is needed in order to use g++ for linking
with TOOLCHAIN=llvm.

Makefile.include

@@ -310,7 +310,7 @@ LINKFLAGPREFIX ?= -Wl,
 
 DIRS += $(EXTERNAL_MODULE_DIRS)
 
-_LINK = $(if $(CPPMIX),$(CXX),$(LINK)) $(UNDEF) $(LINKFLAGPREFIX)--start-group $(BASELIBS) -lm $(LINKFLAGPREFIX)--end-group  $(LINKFLAGPREFIX)-Map=$(BINDIR)/$(APPLICATION).map $(LINKFLAGS)
+_LINK = $(if $(CPPMIX),$(LINKXX),$(LINK)) $(UNDEF) $(LINKFLAGPREFIX)--start-group $(BASELIBS) -lm $(LINKFLAGPREFIX)--end-group  $(LINKFLAGPREFIX)-Map=$(BINDIR)/$(APPLICATION).map $(LINKFLAGS)
 
 ifeq ($(BUILD_IN_DOCKER),1)
 link: ..in-docker-container
@@ -320,7 +320,7 @@ link: ..compiler-check ..build-message $(RIOTBUILD_CONFIG_HEADER_C) $(USEPKG:%=$
 	$(Q)DIRS="$(DIRS)" "$(MAKE)" -C $(APPDIR) -f $(RIOTMAKE)/application.inc.mk
 ifeq (,$(RIOTNOLINK))
 ifeq ($(BUILDOSXNATIVE),1)
-	$(Q)$(if $(CPPMIX),$(CXX),$(LINK)) $(UNDEF) -o $(ELFFILE) $$(find $(BASELIBS) -size +8c) $(LINKFLAGS) $(LINKFLAGPREFIX)-no_pie
+	$(Q)$(if $(CPPMIX),$(LINKXX),$(LINK)) $(UNDEF) -o $(ELFFILE) $$(find $(BASELIBS) -size +8c) $(LINKFLAGS) $(LINKFLAGPREFIX)-no_pie
 else
 	$(Q)$(_LINK) -o $(ELFFILE)
 endif

makefiles/toolchain/gnu.inc.mk

@@ -8,6 +8,7 @@ export AR         = $(PREFIX)ar
 endif
 export AS         = $(PREFIX)as
 export LINK       = $(PREFIX)gcc
+export LINKXX     = $(PREFIX)g++
 export SIZE       = $(PREFIX)size
 export OBJCOPY   ?= $(shell command -v $(PREFIX)objcopy gobjcopy objcopy | head -n 1)
 ifeq ($(OBJCOPY),)

makefiles/toolchain/llvm.inc.mk

@@ -17,6 +17,7 @@ export NM          = $(LLVMPREFIX)nm
 # `makefiles/libc/newlib.mk` are not supported. Therefore
 # we just use GCC for now.
 export LINK        = $(PREFIX)gcc
+export LINKXX      = $(PREFIX)g++
 # objcopy does not have a clear substitute in LLVM, use GNU binutils
 #export OBJCOPY     = $(LLVMPREFIX)objcopy
 export OBJCOPY    ?= $(shell command -v $(PREFIX)objcopy gobjcopy objcopy | head -n 1)