tools build tests: Don't hardcode gcc name

Use $(CC) instead of harcoded gcc binary name.
Signed-off-by: David Carrillo-Cisneros <davidcc@google.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Paul Turner <pjt@google.com>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/20170827075442.108534-2-davidcc@google.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/build/tests/ex/Makefile

@@ -8,7 +8,7 @@ ex:
 include $(srctree)/tools/build/Makefile.include
 
 ex: ex-in.o libex-in.o
-	gcc -o $@ $^
+	$(CC) -o $@ $^
 
 ex.%: fixdep FORCE
 	make -f $(srctree)/tools/build/Makefile.build dir=. $@