add support for building arm64 binaries

scripts/build.sh

@@ -12,7 +12,7 @@ DIR="$( cd -P "$( dirname "$SOURCE" )/.." && pwd )"
 cd $DIR
 
 # Determine the arch/os combos we're building for
-XC_ARCH=${XC_ARCH:-"386 amd64 arm"}
+XC_ARCH=${XC_ARCH:-"386 amd64 arm arm64"}
 XC_OS=${XC_OS:-linux darwin windows freebsd openbsd}
 
 # Delete the old dir
@@ -27,6 +27,7 @@ set +e
 gox \
     -os="${XC_OS}" \
     -arch="${XC_ARCH}" \
+    -osarch="!darwin/arm !darwin/arm64" \
     -ldflags "${GOLDFLAGS}" \
     -output "pkg/{{.OS}}_{{.Arch}}/packer" \
     .