fix: never use libunwind for libc++abi

As noted in Debian Bug 1108414 amd 1071210, there seems no sensible way to use libunwind and gcc's unwind together.

There will always be instances where those two libraries call into each other, depending on the order the libraries are loaded, architecture details in regards to unwinding, and glibc potentially use dlopen to load libgcc_s.so.

TLDR, do this for a segfault:

cat >/tmp/test.cpp << EOF
#include <pthread.h>
#include <thread>
int main()
{
    std::thread systhr([]() { std::this_thread::sleep_for(std::chrono::seconds(10000)); });

    std::this_thread::sleep_for(std::chrono::seconds(1));
    pthread_cancel(systhr.native_handle());

    systhr.join();
    return 0;
}
EOF

clang++ -stdlib=libc++ -o /tmp/test /tmp/test.cpp && /tmp/test

Creating this MR, since apparently no one looks at the Bugreports.