selftests/futex: Fix build error with OUTPUT variable

The use of $$OUTPUT in the target shell commands resulted in an empty
string followed by an absolute path for which mkdir failed:

$ make -C tools/testing/selftests/futex
make: Entering directory '/home/dvhart/source/linux/linux-pdx86/tools/testing/selftests/futex'
Makefile:36: warning: overriding recipe for target 'clean'
../lib.mk:55: warning: ignoring old recipe for target 'clean'
for DIR in functional; do		\
	BUILD_TARGET=$OUTPUT/$DIR;	\
	mkdir $BUILD_TARGET  -p;	\
	make OUTPUT=$BUILD_TARGET -C $DIR all;\
done
mkdir: cannot create directory ‘/functional’: Permission denied

Replace $$OUTPUT with $(OUTPUT) when referring to the Makefile OUTPUT
variable. The above make command now completes successfully.

Fixes: a8ba798b ("selftests: enable O and KBUILD_OUTPUT")
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>
Cc: linux-kselftest@vger.kernel.org
Cc: bamvor.zhangjian@huawei.com
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>