[ItaniumCXXABI] Don't mark an extern_weak init function as dso_local on windows

Since 6bf108d7, we try to not mark extern_weak symbols as
dso_local, to allow using COFF stubs for references to those symbols
(as the symbol may be missing, resolving to an absolute address zero,
outside of the current DSO).

Differential Revision: https://reviews.llvm.org/D71716