[llvm-objcopy] Rename --keep to --keep-section.

Summary:
llvm-objcopy/strip support `--keep` (for sections) and `--keep-symbols` (for symbols). For consistency and clarity, rename `--keep` to `--keep-section`.
In fact, for GNU compatability, -K is --keep-symbol, so it's weird that the alias `-K` is not the same as the short-ish `--keep`.

Reviewers: jakehehrlich, jhenderson, alexshap, MaskRay, espindola

Reviewed By: jakehehrlich, MaskRay

Subscribers: emaste, arichardson, llvm-commits

Differential Revision: https://reviews.llvm.org/D54477

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@346782 91177308-0d34-0410-b5e6-96231b3b80d8
11 files changed