[ELF] Support response file.

Response file is a command line argument in the form of @file. The GNU-
compatible driver expands the file contents, replacing @file argument.

Differential Revision: http://llvm-reviews.chandlerc.com/D3210

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@205038 91177308-0d34-0410-b5e6-96231b3b80d8