Bug#1071210: libc++abi1-18:amd64: libc++abi probably should not use libunwind, seqfaults with pthread_cancel

[Norbert Lange]

Found some discussion, kinda reinforcing my assumption:
https://reviews.llvm.org/D106703

ie. you should only ever have one of the unwinder libraries on one
system, the other should
be not there or a symlink.