[ELF] Support --defsym option to define an absolute symbol.

This patch is to support --defsym option for ELF file format/GNU-compatible
driver. Currently it takes a symbol name followed by '=' and a number. If such
option is given, the driver sets up an absolute symbol with the specified
address. You can specify multiple --defsym options to define multiple symbols.

GNU LD's --defsym provides many more features. For example, it allows users to
specify another symbol name instead of a number to define a symbol alias, or it
even allows a symbol plus an offset (e.g. --defsym=foo+3) to define symbol-
relative alias. This patch does not support that, but will be supported in
subsequent patches.

Differential Revision: http://llvm-reviews.chandlerc.com/D3208

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@205029 91177308-0d34-0410-b5e6-96231b3b80d8
Attempt to unbreak buildbots.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@205034 91177308-0d34-0410-b5e6-96231b3b80d8
[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