selftest/mount: 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/mount/Makefile

 # Makefile for mount selftests.
-
+CFLAGS = -Wall \
+         -O2
 all: unprivileged-remount-test
 
 unprivileged-remount-test: unprivileged-remount-test.c
-	gcc -Wall -O2 unprivileged-remount-test.c -o unprivileged-remount-test
+	$(CC) $(CFLAGS) unprivileged-remount-test.c -o unprivileged-remount-test
 
 include ../lib.mk