[PECOFF] Win64 DLL entry function name is _DllMainCRTStartup.

Unlike Win32/x86, it has no "@12" suffix.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@206648 91177308-0d34-0410-b5e6-96231b3b80d8
Fix broken test.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@206651 91177308-0d34-0410-b5e6-96231b3b80d8