Fix the check for whether we should use `_objc_msgForward_stret`.

I'm not sure if erikdoe/ocmock was mistaken to originally use `__LP64__`
as a proxy for `_objc_msgForward_stret`'s availability or the SDK changed,
but as of the 7.1 SDK `_objc_msgForward_stret` is available unless compiling
for ARM64.