[clang-offload-bundler] Add sections and set section flags using one llvm-objcopy invocation

llvm-objcopy has been changed to support adding a section and updating section flags
in one run (D90438), so we can now change clang-offload-bundler to run llvm-objcopy
tool only once when creating fat object.

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D102670