Add ARM64 & X86_64 Assembly, plus tests

This adds assembly code or removes UNTESTED annotation from
TWO_ARG_DOWNCALLand THREE_ARG_DOWNCALL macros and supporting code,
generating working allocation stubs.

Some object and array allocation tests are added to the stub_test.

Change-Id: I5e93b7543c1e6dbd33b0d4cf564c7cbd963e74ef