ART: Add instrumentation stubs for ARM64 and X86-64

Adds instrumentation stubs necessary for debugger support.

Refactors MethodAndCode to a top-level TwoWordReturn. A function
having a return type of TwoWordReturn will return its two-word
content, either 2x32b or 2x64b, in two registers according to
the architecture's ABI.

Bug: 15443938
Change-Id: Id7e1fbd4ad8eb6f29e23d48903c76f77b28d981a