[AIX] Avoid depending on objdump

On AIX, we are currently relying on `objdump` to detemine the
is_32_bit_windows.

`objdump` is not installed by default on AIX, it is creating problem to
depend on it, especially just for an always false detection on AIX.

So this patch simply provide a always false function to satify the
detection.

Reviewed By: #powerpc, daltenty

Differential Revision: https://reviews.llvm.org/D112952