armvirt: merge arm64 as subtarget 64

Original armvirt target is now subtarget 32.  Other than that the built
result should remain the same as before

Besides old features already available with arm64, the new armvirt/64
subtarget will also have those features originally enabled for
armvirt/32 with commit 44ecfc2 ("armvirt: new target")
 - pl011, uart
 - pl031, rtc
 - pl061, gpio
 - pci-host-generic
 - virtio_{mmio,pci,net,blk,scsi,9p,console,balloon}
 - smp with NR_CPUS=4
 - cpu-hotplug
 - ext4
 - DEBUG_BUGVERBOSE for debug purposes

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>