vboot_reference: fix the build for arm64
vboot_reference: fix the build for arm64

Add "arm64" support to Makefile to use crossystem_arch.c
implementation from host/arm/lib directory, in order to avoid
the code duplication.

BUG=None
TEST='emerge-arm64-generic vboot_reference' works correctly
BRANCH=None

Change-Id: I349f8b2055c9be6ebaeb6f322e3b22260465dd5a
Reviewed-on: https://chromium-review.googlesource.com/1082195
Commit-Ready: Adam Kallai <cbca0ee69738ce83f464ab95304552275ec539d3@inf.u-szeged.hu>
Tested-by: Adam Kallai <cbca0ee69738ce83f464ab95304552275ec539d3@inf.u-szeged.hu>
Reviewed-by: Randall Spangler <62698fdbb84d1779579ee80c3f39fac22017e5bc@chromium.org>
Reviewed-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

coreboot/vboot,coreboot/vboot,coreboot/vboot,coreboot/vboot,coreboot/vboot