Starting on macOS 11, the dynamic loader has a builtin cache of
system-provided dylib files.  They do not actually exist on the
filesystem.  However, runtime dependencies recorded in Mach-O binaries
can still have `LC_LOAD_DYLIB` entries referring to such dylib files by
absolute path.  The dynamic loader simply resolves the paths from its
cache.  Teach `file(GET_RUNTIME_DEPENDENCIES)` to skip dependencies on
such dylib paths.  For practical software distribution purposes they do
not exist, or at least can be assumed available on all deployments.

Issue: #20863