arm64: Add support for the architecture

arm64: Add support for the architecture

This changeset adds basic support to run HelenOS on AArch64, targeting
the QEMU virt platform.

Boot:
* Boot relies on the EDK II firmware, GRUB2 for EFI and the HelenOS
  bootloader (UEFI application). EDK II loads GRUB2 from a CD, GRUB2
  loads the HelenOS bootloader (via UEFI) which loads OS components.
* UEFI applications use the PE/COFF format and must be relocatable. The
  first problem is solved by manually having the PE/COFF headers and
  tables written in assembler. The relocatable requirement is addressed
  by compiling the code with -fpic and having the bootloader relocate
  itself at its startup.

Kernel:
* Kernel code for AArch64 consists mostly of stubbing out various
  architecture-specific hooks: virtual memory management, interrupt and
  exception handling, context switching (including FPU lazy switching),
  support for virtual timer, atomic sequences and barriers, cache and
  TLB maintenance, thread and process initialization.
* The patch adds a kernel driver for GICv2 (interrupt controller).
* The PL011 kernel driver is extended to allow userspace to take
  ownership of the console.
* The current code is not able to dynamically obtain information about
  available devices on the underlying machine. The port instead
  implements a machine-func interface similar to the one implemented by
  arm32. It defines a machine for the QEMU AArch64 virt platform. The
  configuration (device addresses and IRQ numbers) is then baked into
  the machine definition.

User space:
* Uspace code for AArch64 similarly mostly implements
  architecture-specific hooks: context saving/restoring, syscall
  support, TLS support.

The patchset allows to boot the system but user interaction with the OS
is not yet possible.