Add x86_64-*-openbsd* as synonym for amd64-*-openbsd.
Which one you get depends on which config.sub we have, and newer
(2004-*) config.subs normalise all amd64s to x86_64s.
HostVendor_CPP='unknown'
HostOS_CPP='openbsd'
;;
-amd64*-*-openbsd*)
+amd64-*-openbsd*|x86_64-*-openbsd*)
HostPlatform=x86_64-unknown-openbsd
TargetPlatform=x86_64-unknown-openbsd
BuildPlatform=x86_64-unknown-openbsd