[ItaniumCXXABI] Make tls wrappers properly comdat

Just marking a symbol as weak_odr/linkonce_odr isn't enough for
actually tolerating multiple copies of it at linking on windows,
it has to be made a proper comdat; make it comdat for all platforms
for consistency.

This should hopefully fix
https://bugzilla.mozilla.org/show_bug.cgi?id=1566288.

Differential Revision: https://reviews.llvm.org/D71572