ART: Let mterp know about invoke-custom

By default, the mterp assembly interpreter will redirect any
new or unexpected dex opcode to the reference switch interpreter.
That's what is happening now with the new invoke-custom opcodes.
This CL simply makes that fallback transition explicit.

Test: ART_TEST_INTERPRETER=true m test-art-host
Change-Id: I9e1388ce58cb6e0618697748c6bafa55fdbbeb57