build: Retire LLVM_CXXFILT option

[emaste]

Summary

The LLVM_CXXFILT option was added when we used ELF Tool Chain tools by
default, because ELF Tool Chain's c++filt failed to demangle some
symbols.  It became the default in commit 9fa94e1c099d ("Turn on
WITH_LLVM_CXXFILT by default").

We now use LLVM binutils by default, and we can simplify the build logic
by moving llvm-cxxfilt under that build option.

[emaste]

src.conf.5 will be regenerated upon commit

[dim]

LGTM