[CFT] tinderbox/universe building clang once

https://people.freebsd.org/~bdrewery/patches/universe-one-clang.diff

This will build clang once if any of the targets specified (or
defaulted) require bootstrapping clang.

It probably has some issues with LLD_BOOTSTRAP in some cases. It could
be improved more in the future for reusing more of the tools built but I
think this is good enough for now as it saves the majority of the time
in the bootstrap phases on clang.

This won't work for GCC unless it learns convenient -target support.
Its needed --sysroot support was also broken until some recent work from
John Baldwin but I'm not sure if that has been committed yet.

Also FYI WITH_SYSTEM_LINKER support is now in to avoid building libclang
for lld on archs that have LLD_BOOTSTRAP set.

I'm putting this out for testing since tinderbox/universe take so long
and I can't possibly test all workflows with it myself.

-- 
Regards,
Bryan Drewery