Revert "Reland: Clean up implicit fall through."

This reverts commit 6208b9aa9c4e8b7c9ff8063965c9f9aa519c1740.

Upstream coreboot has raised concerns that relying on GCC 7+ features
for host utilities is too restrictive, so revert this and go back to
customizing fallthrough annotations by compiler. Cleaned out some of the
C++-specific stuff because vboot isn't built with C++.

BRANCH=None
BUG=None
TEST=Built with clang and GCC.

Change-Id: I75d796d289b0a6c249fc8ac2dadb1453be468642
Signed-off-by: Julius Werner <jwerner@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/2547821
Reviewed-by: Manoj Gupta <manojgupta@chromium.org>
Reviewed-by: Joel Kitching <kitching@chromium.org>
7 files changed