[PECOFF] /export accepts non-decorated symbols.

You usually have to specify the exact name of a symbol to export it,
but for stdcall functions you can omit the @numbers suffix.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@207491 91177308-0d34-0410-b5e6-96231b3b80d8