[vm/ffi] Clean up dealing with tearoffs of asFunction/lookupFunction #42260
Description
The tearoffs of asFunction and lookupFunction contain a generic call to their respective definitions. This triggers our extra checks that we do not have generic calls to these extension methods. To Work around this we've been excluding dart:ffi call sites. However, we do have other call sites in dart:ffi which could be checked.
The cleaner solution would be to exclude the bodies of those specific tearoffs, instead of all code in dart:ffi.