[AIX][llvm-ar] Use the Correct Kind for Bitcode File Inputs

On AIX, when the input files are LLVM bitcode files, `llvm-ar` should set the archive kind to `K_AIXBIG` as well, instead of leaving it to the default `K_GNU`.

Reviewed By: daltenty

Differential Revision: https://reviews.llvm.org/D149377