sys/arduino: work around llvm-ar bug.

llvm-ar behaves weidly when creating thin archive. This only manifests
itself when using arduino sketches as these are built from the "bin"
directory.

Specifically, given a directory "m" and an object in "m/obj.o " an
invocation with CWD==m:

```
llvm-ar rcTs ../m.a obj.o
```

Will create a maformed archive. Binutils does not have any issue with this.

The following command, executed with CWD==m/.. works:

```
llvm-ar rcTs m.a m/obj.o
```

The trick used in this commit is to put the source files in a different
directory than the object files and compile from there.

sys/arduino/Makefile.include

@@ -2,7 +2,7 @@
 
 # Define application sketches module, it will be generated into $(BINDIR)
 SKETCH_MODULE     ?= arduino_sketches
-SKETCH_MODULE_DIR ?= $(BINDIR)/$(SKETCH_MODULE)
+SKETCH_MODULE_DIR ?= $(BINDIR)/$(SKETCH_MODULE)_src
 SKETCHES           = $(wildcard $(APPDIR)/*.sketch)
 include $(RIOTBASE)/sys/arduino/sketches.inc.mk
 

sys/arduino/sketches.inc.mk

@@ -19,7 +19,8 @@ SKETCH_GENERATED_FILES = $(SKETCH_MODULE_DIR)/Makefile $(SKETCH_MODULE_DIR)/$(SK
 # Building the module files
 #   Do not use $^ in receipes as Makefile is also a prerequisite
 $(SKETCH_MODULE_DIR)/Makefile: $(SKETCH_MODULE_DIR)/$(SKETCH_CPP)
-	$(Q)echo 'SRCXX = $(SKETCH_CPP)'               > $@
+	$(Q)echo 'MODULE = $(SKETCH_MODULE)'           > $@
+	$(Q)echo 'SRCXX = $(SKETCH_CPP)'              >> $@
 	$(Q)echo 'include $$(RIOTBASE)/Makefile.base' >> $@
 $(SKETCH_MODULE_DIR)/$(SKETCH_CPP): $(SKETCHES_ALL)
 	@mkdir -p $(@D)