Adjust modulemap to mark mm3dnow as textual header. (#107155)
This avoids issuing the deprecation diagnostic when building the module.
Not building it into a module shouldn't cause any negative impacts,
since it no longer has any declarations other than the header guard.
It's also very rarely included by anything.
Addresses
https://github.com/llvm/llvm-project/pull/96246#issuecomment-2322453809