[ffigen] Only use objc_msgSend variants on x64 (#836)

`objc_msgSend_stret` and `objc_msgSend_fpret` are only available on x64, so just use the normal `objc_msgSend` on arm64.

We can only check the ABI at runtime, so for the stret and fpret variants we need to emit both the variant and the normal `objc_msgSend` function. Then we can decide which to use when the method is invoked at runtime.

This runtime check is complicated by the fact that `objc_msgSend_stret` has a different signature than `objc_msgSend` has for the same method. This is because `objc_msgSend_stret` takes a pointer to the return type as its first arg. If it wasn't for this signature difference we could just change the function name string we pass to `DynamicLibrary.lookup`.

Fixes #829

pkgs/ffigen/CHANGELOG.md

@@ -5,6 +5,7 @@ bindings to **not** be generated by default, since it may result in invalid
 bindings if the compiler makes a wrong guess. A flag `--ignore-source-errors` (or yaml config `ignore-source-errors: true`)
 must be passed to change this behaviour.
 - __Breaking change__: Stop generating setters for global variables marked `const` in C.
+- Fix objc_msgSend being used on arm64 platforms where it's not available.
 
 ## 10.0.0