CHROMIUM: Use IS_ENABLED(CONFIG_ARM64) as the guard

`boot_delta` variable is not used for x86 boards and it causes a build
error. This commit resolves the problem by using IS_ENABLED to show the
variable for X86 devices, too.

BUG=None
TEST=Checked buid succeeds for a x86 board

Signed-off-by: Koki Ryu <kokiryu@google.com>
Change-Id: Ifae548845dda9cfeffe705358c7cb6b173faa51f
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/4197054


Tested-by: Koki Ryu <kokiryu@chromium.org>
Reviewed-by: Hikaru Nishida <hikalium@chromium.org>
Commit-Queue: Koki Ryu <kokiryu@chromium.org>
Signed-off-by: Richard Fung <richardfung@google.com>