[gn build] Add build file for llvm-objcopy

Needed by check-lld.

This should've been part of r349486 but I messed up.

Differential Revision: https://reviews.llvm.org/D55831


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@349598 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed