[arm] Support building as a Linux userspace binary for AArch64

[arm] Support building as a Linux userspace binary for AArch64

Add support for building as a Linux userspace binary for AArch64.
This allows the self-test suite to be more easily run for the 64-bit
ARM code.  For example:

  # On a native AArch64 system:
  #
  make bin-arm64-efi/tests.linux && ./bin-arm64-efi/tests.linux

  # On a non-AArch64 system (e.g. x86_64) via cross-compilation,
  # assuming that kernel and glibc headers are present within
  # /usr/aarch64-linux-gnu/sys-root/:
  #
  make bin-arm64-linux/tests.linux CROSS=aarch64-linux-gnu- && \
  qemu-aarch64 -L /usr/aarch64-linux-gnu/sys-root/ \
               ./bin-arm64-linux/tests.linux

Signed-off-by: Michael Brown <mcb30@ipxe.org>