Generate the ART boot image for host as well (used by ART tests).

This will allow us to remove the core.art image compiled in make.

Test: lunch aosp_x86_64 && m test-art-host-gtest
Bug: 147817558
Bug: 147819342
Bug: 156649984
Change-Id: I864f72198d875dc1421c9c029a683bafbe7b71ac
Merged-In: I864f72198d875dc1421c9c029a683bafbe7b71ac
(cherry picked from commit 7f8dac12)