Start ARM64 support: build LuaJIT for arm64-v8a

I don't use the buildandroid.sh for this because it forces the host
compiler to 32-bit (-m32), but we do really need 64-bit host compiler to
build LuaJIT for ARM64 (also I'm in Windows).

So, I used Android NDK standalone toolchain then invoke make like this:
make CROSS=aarch64-linux-android- TARGET_SYS=Linux TARGET_CFLAGS="-D__ANDROID_API__=21" amalg

Then the resulting libluajit.a is copied to android/arm64-v8a/libluajit.a