[Clang][Bundler] Use llvm-objcopy for creating fat object files

clang-offload-bundler currently uses partial linking for creating fat object files, but such technique cannot be used on Windows due to the absence of partial linking support in the linker. This patch changes implementation to use llvm-objcopy for merging device and host objects instead of doing partial linking. This is one step forward towards enabling OpenMP offload on Windows.

Differential Revision: https://reviews.llvm.org/D66485

llvm-svn: 369955