[libcxxabi] link abort_message into unittest_demangle

unittest_demangle.pass.cpp uses the preprocessor to #include
cxa_demangle.cpp. D148566 will make more use of std::string_view in
libcxxabi rather than the home-grown StringView, but as a result of
D149092, a definition of abort_message needs to be provided.

Otherwise builds of check-cxxabi with -DLLVM_ENABLE_ASSERTIONS=ON will
fail to link with the errors:
/usr/bin/ld: /tmp/lit-tmp-0akcq37p/cc6DLdvw.o: in function `(anonymous namespace)::itanium_demangle::starts_with(std::__1::basic_string_view<char, std::__1::char_traits<char> >, char)':
unittest_demangle.pass.cpp:(.text+0x81): undefined reference to `abort_message'
/usr/bin/ld: /tmp/lit-tmp-0akcq37p/cc6DLdvw.o: in function `(anonymous namespace)::itanium_demangle::starts_with(std::__1::basic_string_view<char, std::__1::char_traits<char> >, std::__1::basic_string_view<char, std::__1::char_traits<char> >)':
unittest_demangle.pass.cpp:(.text+0x2aa): undefined reference to `abort_message'
/usr/bin/ld: u...