ART: Fix MonitorExit code on ARM

We do not emit barriers on non-SMP systems. But on ARM, we have
places that need to conditionally execute, which is done through
an IT instruction. The guide of said instruction thus changes
between SMP and non-SMP systems.

To cleanly approach this, change the API so that GenMemBarrier
returns whether it generated an instruction. ARM will have to
query the result and update any dependent IT.

Throw a build system error if TARGET_CPU_SMP is not set.

Fix runtime/Android.mk to work with new multilib host.

Bug: 14989275
Change-Id: I9e611b770e8a1cd4ca19367d7dae0573ec08dc61