[clang] [clang] Add -mlarge-data-threshold for x86_64 medium code model (PR #66839)

[James Y Knight via cfe-commits]

https://github.com/jyknight approved this pull request.

Ok. We should change the default to a non zero value to match gcc, too, but that can be a follow-up.

https://github.com/llvm/llvm-project/pull/66839