Add the platform conditional and set up other basics for the toolchain.
The ConditionalCompilation tests are updated to match, since otherwise
they seem to trip when building on non-OpenBSD platforms. The
Driver/linker test is updated to ensure lld is passed on this platform.
Note that OpenBSD calls "x86_64" as "amd64", so we use that name for the
architecture instead of trying to alias one to the other, as this makes
things simpler.
0d3b92dfb1