host: supp_plugin: fix cross compilation

In host/supp_plugin/Makefile, CC is assigned the cross compiler set in
the Makefile only if the CC is not assigned any value previously. But
according to the GNU make implicit variables, CC will have a default
value of cc and hence the cross compiler set in the Makefile will not be
used while compiling the supp_plugin using makefiles.
"--no-builtin-variables" will cancel all variables used by implicit
rules.

Signed-off-by: Davidson K <davidson.kumaresan@arm.com>
Acked-by: Jerome Forissier <jerome.forissier@linaro.org>
diff --git a/Makefile b/Makefile
index cf80ab1..5c2f11f 100644
--- a/Makefile
+++ b/Makefile
@@ -49,6 +49,7 @@
 .PHONY: test_plugin
 test_plugin:
 	$(q)$(MAKE) -C host/supp_plugin CROSS_COMPILE="$(CROSS_COMPILE_HOST)" \
+			     --no-builtin-variables \
 			     O=$(out-dir)
 
 .PHONY: clean