Change to JITDefault code model for ELF targets

On x86-64 platforms, the small code model assumes that code will be loaded below the 2GB boundary.  With the static relocation model, the fact that the expression code is initially loaded (in the LLDB debugger address space) above that boundary causes problems.  Switching to the JITDefault code model causes the large code model to be used for 64-bit targets and small code model of 32-bit targets.