linkage_checker: handle system libraries on Big Sur

[Bo98]

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From macOS Big Sur 11 Beta Release Notes:

New in macOS Big Sur 11 beta, the system ships with a built-in dynamic linker cache of all system-provided libraries. As part of this change, copies of dynamic libraries are no longer present on the filesystem. Code that attempts to check for dynamic library presence by looking for a file at a path or enumerating a directory will fail. Instead, check for library presence by attempting to dlopen() the path, which will correctly check for the library in the cache. (6298628)

[MikeMcQuaid]

Wow, surprisingly easy. Well done @Bo98 👏

[Bo98]

Thank goodness Fiddle existed!