Fix setting and default setting of code model for jit. Do this

Fix setting and default setting of code model for jit. Do this
by allowing backends to override routines that will default
the JIT and Static code generation to an appropriate code model
for the architecture.

Should fix PR 5773.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@91824 91177308-0d34-0410-b5e6-96231b3b80d8