[llvm-objcopy][ELF] Allow setting SHF_EXCLUDE flag for ELF sections

Summary: This patch adds support for setting SHF_EXCLUDE flag for ELF sections.

Reviewers: jhenderson, grimar, MaskRay, mstorsjo, espindola, alexshap, rupprecht

Reviewed By: jhenderson, MaskRay

Subscribers: emaste, abrachet, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72128