Refactor Ruby platform priority condition to its own method

The `Gem::Platform::RUBY ? -1 : 1` has been used multiple times in different places and could be refactored to a method (DRY).

lib/rubygems/name_tuple.rb

@@ -89,9 +89,8 @@ def inspect # :nodoc:
   alias to_s inspect # :nodoc:
 
   def <=>(other)
-    [@name, @version, @platform == Gem::Platform::RUBY ? -1 : 1] <=>
-      [other.name, other.version,
-       other.platform == Gem::Platform::RUBY ? -1 : 1]
+    [@name, @version, Gem::Platform.sort_priority(@platform)] <=>
+      [other.name, other.version, Gem::Platform.sort_priority(other.platform)]
   end
 
   include Comparable

lib/rubygems/platform.rb

@@ -40,6 +40,10 @@ def self.match_gem?(platform, gem_name)
     match_platforms?(platform, Gem.platforms)
   end
 
+  def self.sort_priority(platform)
+    platform == Gem::Platform::RUBY ? -1 : 1
+  end
+
   def self.installable?(spec)
     if spec.respond_to? :installable_platform?
       spec.installable_platform?

lib/rubygems/resolver/installer_set.rb

@@ -71,7 +71,7 @@ def add_always_install(dependency)
     end
 
     found = found.sort_by do |s|
-      [s.version, s.platform == Gem::Platform::RUBY ? -1 : 1]
+      [s.version, Gem::Platform.sort_priority(s.platform)]
     end
 
     newest = found.last

lib/rubygems/specification.rb

@@ -855,7 +855,7 @@ def self._resort!(specs) # :nodoc:
       next names if names.nonzero?
       versions = b.version <=> a.version
       next versions if versions.nonzero?
-      b.platform == Gem::Platform::RUBY ? -1 : 1
+      Gem::Platform.sort_priority(b.platform)
     end
   end
 
@@ -2333,7 +2333,7 @@ def satisfies_requirement?(dependency)
   # Returns an object you can use to sort specifications in #sort_by.
 
   def sort_obj
-    [@name, @version, @new_platform == Gem::Platform::RUBY ? -1 : 1]
+    [@name, @version, Gem::Platform.sort_priority(@new_platform)]
   end
 
   ##