selftests: memfd: Align STACK_SIZE for ARM AArch64 system

The stack size should be 16 bytes aligned in arm64 system. The similar
patch has been merged already.

> <commit id: 1f78dda2>
> selftests: memfd_test: Revised STACK_SIZE to make it 16-byte aligned
>
>    There is a mandate of 16-byte aligned stack on AArch64 [1], so the
>    STACK_SIZE here should also be 16-byte aligned, otherwise we would
>    get an error when calling clone().
>
>    [1] http://lxr.free-electrons.com/source/arch/arm64/kernel/process.c#L265


>
>    Signed-off-by: Chunyan Zhang <zhang.chunyan@linaro.org>
>    Acked-by: Arnd Bergmann <arnd@arndb.de>
>    Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>

Signed-off-by: Orson Zhai <orson.zhai@linaro.org>
Reviewed-by: Sumit Semwal <sumit.semwal@linaro.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>