Spack heuristically adds `<install prefix>/lib` and `<install prefix>/lib64` as rpath entries, as it doesn't know what the install dir is going to be ahead of the build. This PR cleans up non-existing, absolute paths[^1], which
1. avoids redundant stat calls at runtime
2. drops redundant rpaths in `patchelf`, making it relocatable -- you don't need patchelf recursively then.
[^1]: It also removes relative paths not starting with `$` (so, `$ORIGIN/../lib` is retained -- we _could_ interpolate `$ORIGIN`, but that's hard to get right when symlinks have to be taken into account). Relative paths _are_ supported in glibc, but are relative to _the current working directory_, which is madness, and it would be better to drop those paths.
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