selftest/memfd: enable cross compilation

Use the CC variable instead of hard coding gcc. Also clean up the compiler
options by creating a CFLAGS variable.
Signed-off-by: NTyler Baker <tyler.baker@linaro.org>
Signed-off-by: NShuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/memfd/Makefile

+CC = $(CROSS_COMPILE)gcc
 CFLAGS += -D_FILE_OFFSET_BITS=64
 CFLAGS += -I../../../../include/uapi/
 CFLAGS += -I../../../../include/
 
 all:
-	gcc $(CFLAGS) memfd_test.c -o memfd_test
+	$(CC) $(CFLAGS) memfd_test.c -o memfd_test
 
 TEST_PROGS := memfd_test
 
 include ../lib.mk
 
 build_fuse:
-	gcc $(CFLAGS) fuse_mnt.c `pkg-config fuse --cflags --libs` -o fuse_mnt
-	gcc $(CFLAGS) fuse_test.c -o fuse_test
+	$(CC) $(CFLAGS) fuse_mnt.c `pkg-config fuse --cflags --libs` -o fuse_mnt
+	$(CC) $(CFLAGS) fuse_test.c -o fuse_test
 
 run_fuse: build_fuse
 	@./run_fuse_test.sh || echo "fuse_test: [FAIL]"