[rubygems/rubygems] Ensure we are using the same extension dir

Since #6945 the extension dir changed to Gem::BasicSpecification's implementation, we didn't hook that in rubygems_ext.rb. So for universal rubies, we ended up using the universal platform name when installing, but arch replaced platform name when checking. This lead to native extensions can never be correctly installed on universal rubies.

Hook Gem::BasicSpecifications so the behavior is consistent on installing and checking.

https://github.com/rubygems/rubygems/commit/8d699ed096

lib/bundler/rubygems_ext.rb

@@ -338,7 +338,7 @@ module Gem
   end
 
   # On universal Rubies, resolve the "universal" arch to the real CPU arch, without changing the extension directory.
-  class Specification
+  class BasicSpecification
     if /^universal\.(?<arch>.*?)-/ =~ (CROSS_COMPILING || RUBY_PLATFORM)
       local_platform = Platform.local
       if local_platform.cpu == "universal"

spec/bundler/bundler/stub_specification_spec.rb

@@ -19,6 +19,17 @@ RSpec.describe Bundler::StubSpecification do
     end
   end
 
+  describe "#gem_build_complete_path" do
+    it "StubSpecification should have equal gem_build_complete_path as Specification" do
+      spec_path = File.join(File.dirname(__FILE__), "specifications", "foo.gemspec")
+      spec = Gem::Specification.load(spec_path)
+      gem_stub = Gem::StubSpecification.new(spec_path, File.dirname(__FILE__),"","")
+
+      stub = described_class.from_stub(gem_stub)
+      expect(stub.gem_build_complete_path).to eq spec.gem_build_complete_path
+    end
+  end
+
   describe "#manually_installed?" do
     it "returns true if installed_by_version is nil or 0" do
       stub = described_class.from_stub(with_bundler_stub_spec)