Build system recognizes OpenBSD.

Commit for CMake and build scripts to recognize OpenBSD. To keep this
commit relatively short, this just deals with the rather simple and
uncontroversial changes to the build system.

Note that OpenBSD calls "x86_64" as "amd64", Since the Swift stdlib will
be put in a subdirectory named after ARCH, to ensure the standard
library is properly found later, we use the native architecture name for
OpenBSD in the build system rather than trying to deal with the
difference the other way around.

utils/build-script-impl

@@ -404,6 +404,7 @@ function verify_host_is_supported() {
     local host="$1"
     case ${host} in
       freebsd-x86_64            \
+      | openbsd-amd64           \
       | cygwin-x86_64           \
       | haiku-x86_64            \
       | linux-x86_64            \