Add llvm-dlltool to the toolchain list (#72563)

This adds dlltool to the list of tools which don't get excluded from
installation when LLVM_INSTALL_TOOLCHAIN_ONLY is set.

The most important effect here is that this tool will now be included in
the official Windows release.

While llvm-lib reuses the dlltool machinary internally and has many of
the same capabilities, it does not expose the functionality controller
by the '-k' flag, which is currently the only way to create import
libraries for i386 with stdcall symbols from a module definition alone.

We avoid changing llvm-lib tool, since it is designed to emulate LIB.EXE
from MSVC toolchain, and as this functionality is not supported there,
we would have had to introduce an LLVM extension flag in order to
support it.

See https://reviews.llvm.org/D36548 for reference on rationale for
dlltool '-k' flag.