x86: tighten extend-to-32bit-address conditions

x86: tighten extend-to-32bit-address conditions

In a442cac5084e ("ix86: wrap constants") I made the truncation condition
too relaxed: Any indication of a mode that's possible with BFD64 only
should avoid the truncation. Therefore, like in the other two cases of
calls to extend_to_32bit_address(), also check whether we're generating
a 64-bit object.