Add WITH_LLVM_CXXFILT option to install llvm-cxxfilt as c++filt

Since elftoolchain's cxxfilt is rather far behind on features, and we
ran into several bugs, add an option to use llvm-cxxfilt as an drop-in
replacement.

It supports the same options as elftoolchain cxxfilt, though it doesn't
have support for old ARM (C++ Annotated Reference Manual, not the CPU)
and GNU v2 manglings. But these are irrelevant in 2020.

Note: as we already compile the required libraries as part of libllvm,
this will not add any significant build time either.

PR: 250702
Reviewed by: emaste, yuri
Differential Revision: https://reviews.freebsd.org/D27071
MFC after: 2 weeks