[vm,dyn_modules] Add native entry fallbacks for FFI callback creation.

In compiled code, static calls to FFI callback creation functions are
replaced with different IL, so those functions should never be called
from compiled code and so the code assigned to them always errors.

For the creation of callbacks in interpreted JIT code, this CL replaces
the always-erroring definitions with native entries that check that the
caller is interpreted and running in JIT mode and, if so, creates an FFI
callback and returns it. In AOT mode and for other callers, the native
entry returns an error as before.

This CL also weakens the return check for native entries when dynamic
modules are enabled to allow for returning Function objects as well as
Instances. These FFI callback creation functions are only called from
transformed code, and so the returned Function object is consumed in
the transformed code and never returned to user code.

There are still some issues remaining with isolate group bound
callbacks and async callbacks that are exposed by removing the
"This function should be handled at the callsite." error. Those
will be addressed in followups.

TEST=RTE->Pass:
       ffi/function_callbacks
       ffi/isolate_local_function_callbacks
       ffi/vmspecific_function_callbacks
       ffi/native_callables
       (and many more)
     RTE->Crash:
       ffi/async_void_function_callbacks
       ffi/isolate_group_bound_callback
       ffi/vmspecific_handle_test
       ffi/regress_jump_to_frame
       ffi/threading_reentrant_test
     RTE->Timeout:
       ffi/threading_test

Cq-Include-Trybots: luci.dart.try:vm-dyn-linux-debug-x64-try,vm-dyn-mac-debug-arm64-try
Change-Id: I4e5c9d76c2720db8543b835c81d5f317b1811cb9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/525281
Commit-Queue: Tess Strickland <sstrickl@google.com>
Reviewed-by: Alexander Markov <alexmarkov@google.com>