Declares __cpu_model as dso local
builtin_cpu_supports and builtin_cpu_is use information in cpu_model to decide cpu features. Before this change, cpu_model was not declared as dso local. The generated code looks up the address in GOT when reading __cpu_model. This makes it impossible to use these functions in ifunc, because at that time GOT entries have not been relocated. This change makes it dso local.
Differential Revision: https://reviews.llvm.org/D53850