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>