[PECOFF] Support =internalName syntax in .def file.

Export definitions in a module definition file is as follows:

  exportedname[=internalname] [@ordinal [NONAME]] [PRIVATE] [DATA]

Previously we did not support =internalname, so users couldn't export
symbols from a DLL with a different name.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@207827 91177308-0d34-0410-b5e6-96231b3b80d8