Fix String.equals() for moveable String.class.

If the String.class is moveable (i.e. running without boot
image), the instanceof check emitted by the JIT in the
String.equals() intrinsic would require read barriers.
As we do not really care about the performance of running
without the boot image, disable the intrinsic in this case.

Test: 669-moveable-string-class-equals (--jit)
Bug: 68181300
Change-Id: I39c9f9935e0482b3b30f1ae5cd23515cbda0603b