[lld] Fix wrong content permission bit.

Summary: "W" bit value was not consistent in ContentPermissions enum. It should always be 2.

Reviewers: kledzik

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D810

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@182515 91177308-0d34-0410-b5e6-96231b3b80d8