ar_to_bc.sh: Ignore non-bitcode files in archives

The script uses llvm-link to link LLVM bitcode files.
5426da8f used -DLLVM_DISABLE_ASSEMBLY_FILES=ON
to ignore object files compiled from lib/Support/BLAKE3/*.S.

A better approach (which fits Bazel better) is to ignore non-bitcode files.

Reviewed By: akyrtzi

Differential Revision: https://reviews.llvm.org/D126728