[PECOFF] Fix /export option.

MSDN doesn't say about /export:foo=bar style option, but
it turned out MSVC link.exe actually accepts that. So we need that
too.

It also means that the export directive in the module definition
file and /export command line option are functionally equivalent.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@218695 91177308-0d34-0410-b5e6-96231b3b80d8