Templatize `ClassLinker::LinkMethodsHelper`.

Make the pointer size a template parameter to let the C++
compiler better optimize the method linking. Use constant
expressions for `ArtMethod` size and alignment.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Bug: 181943478
Change-Id: Iebfdbac60c33b5194e86be2e73cb8ea5331b2947