llvm-cxxfilt: Install as c++filt only with LLVM_BINUTILS

When building WITHOUT_LLVM_BINUTILS /usr/bin/c++filt is the one from
ELF Tool Chain.

PR: 293610
Fixes: 5d757312ad59 ("build: Retire LLVM_CXXFILT option")
Sponsored by: The FreeBSD Foundation