Makefile: Clean up .gcda files in prof-gen.

The .gcda files generated by "$(MAKE) PROF_FLAGS=-fprofile-generate all"
should be removed. Otherwise, the profile data in them would be combined
with the profile data of user's workload.Then, they would be used by
"make prof-use" later.

Author: Steven Wang

build/Makefile.global

@@ -134,6 +134,7 @@ distclean: clean
 
 prof-gen:
 	CCACHE_DISABLE=1 $(MAKE) PROF_FLAGS=-fprofile-generate all
+	find .  -name \*.gcda | xargs rm -f
 
 prof-clean:
 	find . -name \*.lo -o -name \*.o | xargs rm -f