portability fixes: support building vboot on FreeBSD

Built on FreeBSD 12.1-RELEASE, 13-CURRENT, using gcc9 installed from
packages.

Change-Id: Ifa8bb343c7e916c1b545cf6c1e4bd0a18ea391cd
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/2382790
Reviewed-by: Julius Werner <jwerner@chromium.org>
Tested-by: Julius Werner <jwerner@chromium.org>
Commit-Queue: Julius Werner <jwerner@chromium.org>
13 files changed