[PECOFF] Fix dllexported name.

Previously the handling of exported symbol was wrong if it's
specified in a module definition file in the form of
<externalname>=<internalname>. Export the correct symbol.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@208446 91177308-0d34-0410-b5e6-96231b3b80d8