Maintain only a single libunwind build

We currently have 3 users of libunwind - perf, PyPerf & pyspy/rbspy. Each has its own build on a different system in the Dockerfiles (although there's a build script shared by some).

We can have a single build (and copy the resulting `libunwind.a` to other steps in the Dockerfile). This will help in:
1. Reducing build time (instead of building 3-6 copies, we will build 1, shared between the Dockerfile & pyi.Dockerfile). Actually, maybe at least 2, because one will be musl.
2. Reducing variables - a single maintained build & a single version used.