rusty-gd: add stack start stubs, define shim library

to avoid duplicate symbols, all CXX FFI integrated into the native code
needs to be from a single library. Move init_flags FFI into libbt_shim
too.

Bug: 171749953
Tag: #gd-refactor
Test: gd/cert/run --rhost SimpleHalTest
Change-Id: I5207f4df903cf434b470be84133179b04d5c9483