[AIX][LLVM] Update the default code model for 64-bit (#199301)
This changes the default code model on 64-bit AIX to large.
For many applications, the existing small code model is simply
inadequate to build. Users often then end up relying on expensive linker
fixups (i.e. -Wl,-bigtoc) to work around the issue when they would have
been better served by moving to the larger code model. Our analysis and
benchmarking leads us to believe this's generally a net benefit to
users.
(Assisted by AI)