[llvm-strip] Support --keep and --strip-all-gnu from llvm-objcopy

Summary: Add --keep and --strip-all-gnu from llvm-objcopy into llvm-strip.

Reviewers: jakehehrlich, jhenderson, alexshap

Reviewed By: jhenderson, alexshap

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D53954

llvm-svn: 345861