ART: Refactor intrinsics slow-paths

Refactor slow paths so that there is a default implementation for
common cases (only arm64 with vixl is special). Write a generic
intrinsic slow-path that can be reused for the specific architectures.
Move helper functions into CodeGenerator so that they are accessible.

Change-Id: Ibd788dce432601c6a9f7e6f13eab31f28dcb8550