[PECOFF] Fix /export option in the .drectve section.

/EXPORT option has slightly different semantics if it appears in the .drectve
section. This patch implements it.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@197970 91177308-0d34-0410-b5e6-96231b3b80d8