make: Correct multilib detection when using LLVM

The LLVM makefile needs to be included _after_ the CPU makefiles have
set their CFLAGS in order to get the right multilib directory from GCC.

Makefile.include

@@ -175,9 +175,6 @@ export TOOLCHAIN
 # will most likely not need to touch this.
 export PREFIX ?= $(if $(TARGET_ARCH),$(TARGET_ARCH)-)
 
-# Import all toolchain settings
-include $(RIOTCPU)/Makefile.include.$(TOOLCHAIN)
-
 # Add standard include directories
 INCLUDES += -I$(RIOTBASE)/core/include -I$(RIOTBASE)/drivers/include -I$(RIOTBASE)/sys/include
 INCLUDES += -I$(RIOTCPU)/$(CPU)/include
@@ -189,6 +186,9 @@ include $(RIOTBASE)/Makefile.defaultmodules
 include $(RIOTBOARD)/$(BOARD)/Makefile.include
 include $(RIOTCPU)/$(CPU)/Makefile.include
 
+# Import all toolchain settings
+include $(RIOTCPU)/Makefile.include.$(TOOLCHAIN)
+
 # get number of interfaces straight before resolving dependencies
 GNRC_NETIF_NUMOF ?= 1