[PECOFF] Drop stdcall's atsign suffix only.

You can omit @number suffix when specifying /export option,
but you can do that only for stdcall functions.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@207809 91177308-0d34-0410-b5e6-96231b3b80d8