ART: Make build error a warning for TARGET_CPU_SMP
ART: Make build error a warning for TARGET_CPU_SMP

Builds that do not build for targets will not have this information.
Make it a warning.

Change-Id: Ibd5fa6f5d2c1cf6d86de30df24d25eddb23fb81d

treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst