The issue of clang 6.0.0 crashing when building Wine was resolved with
__FreeBSD_version 1200057, so reduce the window of versions we need a
workaround for to just 1200056.

In the mid term I plan on removing this workaround, but let's keep it
in place for a bit longer for the sake of users on an affected snapshot.

PR: 224863
Thanks to: dim