[Audit] Prune absolute paths starting with `${prefix}` from rpath on Apple too, and make list of rpaths unique

#1186 automatically pruned absolute paths starting with `/workspace` for ELF files.  This should be extended to Mach-O files too.  Also, we should always try to make the list of rpaths unique, apparently that can cause troubles on macOS: https://discourse.julialang.org/t/arb-jll-fails-to-load-precompile-dylib-after-macos-update/127989.