Touch up changes related to cross-dex compiling

 * Added exaplanatory comments
 * Used IsBootStrapClassLoaded to avoid a read barrier
 * Use the new multi-image from CompilerOptions

Bug: 154012332
Test: art/test/testrunner/testrunner.py --host --64 --optimizing -b
Change-Id: I10a4ed0f9d5b2e796102846564a54dc90f297068
5 files changed