armvirt: new target

An ARM Cortex-A15 machine provided by QEMU.

Kernel drivers enabled:

 - 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
 - neon, vfp extensions support (otherwise userland will fail with
   illegal instruction signal (code 0x00000004))
 - DEBUG_BUGVERBOSE for debug purposes

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