Add support for 64-bit kernels

[Zahrun]

I had trouble compiling a kernel module with errors along these lines:

ERROR: modpost: "__const_udelay" [/home/pi/rtl8812au/88XXau.ko] undefined!
ERROR: modpost: "__arch_copy_from_user" [/home/pi/rtl8812au/88XXau.ko] undefined!
ERROR: modpost: "_raw_spin_unlock" [/home/pi/rtl8812au/88XXau.ko] undefined!
ERROR: modpost: "__rcu_read_lock" [/home/pi/rtl8812au/88XXau.ko] undefined!
ERROR: modpost: "__rcu_read_unlock" [/home/pi/rtl8812au/88XXau.ko] undefined!
ERROR: modpost: "cpu_hwcap_keys" [/home/pi/rtl8812au/88XXau.ko] undefined!
ERROR: modpost: "csum_ipv6_magic" [/home/pi/rtl8812au/88XXau.ko] undefined!
ERROR: modpost: "__warn_printk" [/home/pi/rtl8812au/88XXau.ko] undefined!
ERROR: modpost: "arm64_const_caps_ready" [/home/pi/rtl8812au/88XXau.ko] undefined!
ERROR: modpost: "__udelay" [/home/pi/rtl8812au/88XXau.ko] undefined!
ERROR: modpost: "preempt_schedule_notrace" [/home/pi/rtl8812au/88XXau.ko] undefined!
ERROR: modpost: "__arch_copy_to_user" [/home/pi/rtl8812au/88XXau.ko] undefined!

I found out from aircrack-ng/rtl8812au#933 that rpi-source does not support 64-bit kernels.
I was able to compile the module properly by using the rpi-source from #2.
Since that was not merged, and the issue still persists, and to give more visibility to anyone who might face the same thing, I open this issue here.
It might be worthwhile to indicate in README.md that rpi-source does not support 64-bit kernels.