ART: Use reinterpret_cast{32,64}<> when appropriate.

And fix UnstartedRuntime checking for null arguments in all
functions where we now use reinterpret_cast32<>.

This is a follow-up to
    https://android-review.googlesource.com/783607 .

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Test: Pixel 2 XL boots.
Test: m test-art-target-gtest
Test: testrunner.py --target --optimizing
Bug: 117427174
Change-Id: I58f8ad59f70e3fbb1d06aef419cd26555706fa65