ART: Generate path to entrypoints in VisitLoadString for arm64

ARM64 CC boot.oat: 81817968 -> 80806672 (-1.23%)

Generate the code for the medium path in VisitLoadString, not by
creating a slow path and generating with the slow paths.

Change-Id: I45df45ea401157ae3d76a2d15dba35f3aed5593f
Tests: test-art-target on bullhead on master branch, N6P CC booting
1 file changed