Define OS::Mac on Linux

Define MacOS.version, MacOS.full_version, and MacOS::Xcode.version to
Version::NULL on Linux so that brew readall succeeds and Homebrew/brew
can tap Homebrew/core on Linux.

Library/Homebrew/os.rb

@@ -21,6 +21,7 @@ module OS
     end
     PATH_OPEN = "/usr/bin/open".freeze
   elsif OS.linux?
+    require "os/linux"
     ISSUES_URL = "https://github.com/Linuxbrew/brew/wiki/troubleshooting".freeze
     PATH_OPEN = "xdg-open".freeze
   end

Library/Homebrew/os/linux.rb

@@ -0,0 +1,28 @@
+module OS
+  # Define OS::Mac on Linux for formula API compatibility.
+  module Mac
+    module_function
+
+    ::MacOS = self # rubocop:disable Naming/ConstantName
+
+    def prefer_64_bit?
+      Hardware::CPU.is_64_bit?
+    end
+
+    def version
+      Version::NULL
+    end
+
+    def full_version
+      Version::NULL
+    end
+
+    module Xcode
+      module_function
+
+      def version
+        Version::NULL
+      end
+    end
+  end
+end