[llvm-ar] Use target triple to deduce archive kind for bitcode inputs

Summary:
When using full LTO on cross-compile settings, instead of generating the
default archive kind of the host platform, we could deduce the archive
kind based on the target triple.

This specifically addresses https://github.com/android/ndk/issues/1209
by making it possible to drop llvm-ar in place of GNU ar without extra
flags.

Reviewers: compnerd, pcc, srhines, danalbert

Subscribers: hiraditya, MaskRay, steven_wu, dexonsmith, rupprecht, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76461