[PECOFF] Add /dllexport option.

/DLLEXPORT is a command line option to export a symbol. __declspec(dllexport)
uses that to make the linker to export DLLExport'ed functions, by adding the
option to .drectve section.

This patch implements the parser of the command line option.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@197122 91177308-0d34-0410-b5e6-96231b3b80d8