Revert "Clean up implicit fall through."

This reverts commit 6a703f5b0edfdbf2eee72d59499f73585262a46f.

Reason for revert: This change breaks building coreboot

Original change's description:
> Clean up implicit fall through.
> 
> Directly use the __attribute__ ((fallthrough)) instead of
> a macro.
> This was suggested in CL:1772474.
> 
> BUG=chromium:997709
> TEST=CQ
> BRANCH=None
> 
> Change-Id: Ic6cd417d7c735395a4b136dbb0879a6f1716da98
> Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/1897360
> Tested-by: Manoj Gupta <manojgupta@chromium.org>
> Reviewed-by: Julius Werner <jwerner@chromium.org>
> Commit-Queue: Manoj Gupta <manojgupta@chromium.org>

Bug: chromium:997709
Change-Id: Icd0de8bcdee44d1b41f313a4f5aaba8108f734ab
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/1899735
Reviewed-by: Mathew King <mathewk@chromium.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>
Commit-Queue: Julius Werner <jwerner@chromium.org>
Tested-by: Julius Werner <jwerner@chromium.org>
6 files changed