[test] [llvm-objcopy] Fix broken test case

Summary: The test case added in D62718 did not work unless the user was root because write bits were not set for the output file. This change uses only permissions with user write (0200) to ensure tests pass regardless of the users permissions.

Reviewers: jhenderson, rupprecht, MaskRay, espindola, alexshap

Reviewed By: MaskRay

Subscribers: emaste, arichardson, jakehehrlich, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64302

llvm-svn: 366527