linkage_checker: handle system libraries on Big Sur

Library/Homebrew/linkage_checker.rb

@@ -3,6 +3,7 @@
 require "keg"
 require "formula"
 require "linkage_cache_store"
+require "fiddle"
 
 class LinkageChecker
   attr_reader :undeclared_deps
@@ -125,6 +126,11 @@ class LinkageChecker
 
           if (dep = dylib_to_dep(dylib))
             @broken_deps[dep] |= [dylib]
+          elsif MacOS.version >= :big_sur && dylib_found_via_dlopen(dylib)
+            # If we cannot associate the dylib with a dependency, then it may be a system library.
+            # In macOS Big Sur and later, system libraries do not exist on-disk and instead exist in a cache.
+            # If dlopen finds the dylib, then the linkage is not broken.
+            @system_dylibs << dylib
           else
             @broken_dylibs << dylib
           end
@@ -151,6 +157,13 @@ class LinkageChecker
   end
   alias generic_check_dylibs check_dylibs
 
+  def dylib_found_via_dlopen(dylib)
+    Fiddle.dlopen(dylib).close
+    true
+  rescue Fiddle::DLError
+    false
+  end
+
   def check_formula_deps
     filter_out = proc do |dep|
       next true if dep.build?