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 <kadam@inf.u-szeged.hu>
Tested-by: Adam Kallai <kadam@inf.u-szeged.hu>
Reviewed-by: Randall Spangler <rspangler@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>

Author: kaadam

Makefile

@@ -103,7 +103,7 @@ endif
 
 ifeq (${ARCH},armv7l)
   override ARCH := arm
-else ifeq (${ARCH},aarch64)
+else ifneq (,$(filter aarch64 arm64,${ARCH}))
   override ARCH := arm
 else ifeq (${ARCH},i386)
   override ARCH := x86