nonmoving: Use correct info table pointer accessor

Previously we used INFO_PTR_TO_STRUCT instead of
THUNK_INFO_PTR_TO_STRUCT when looking at a thunk. These two happen to be
equivalent on 64-bit architectures due to alignment considerations
however they are different on 32-bit platforms. This lead to Trac #17487.

To fix this we also employ a small optimization: there is only one thunk
of type WHITEHOLE (namely stg_WHITEHOLE_info). Consequently, we can just
use a plain pointer comparison instead of testing against info->type.