[llvm-objcopy] Change handling of output file permissions

Summary: Address bug [[ https://bugs.llvm.org/show_bug.cgi?id=42082 | 42082 ]] where files were always outputted with 0775 permissions. Now, the output file is given either 0666 or 0777 if the object is executable.

Reviewers: espindola, alexshap, rupprecht, jhenderson, jakehehrlich, MaskRay

Reviewed By: rupprecht, jhenderson, jakehehrlich, MaskRay

Subscribers: emaste, arichardson, jakehehrlich, MaskRay, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62718

llvm-svn: 365162