[PECOFF] Change the DLL entry symbol.

I'm not 100% sure but it looks like DLL entry symbol (DLL initializer function
name) should be _DllMainCRTStartup@12. The reason why I'm not very sure is
because I have no idea what "@12" suffix is, but without it the symbol won't
be resolved...

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@198072 91177308-0d34-0410-b5e6-96231b3b80d8