[M68k] implement large code model

[TechnoElf]

The currently supported code models all assume that any symbol may be reached with a 16bit offset, however, m68k-based microcontrollers may have ROM/flash (where .text and .rodata are usually mapped) and RAM (where .data and .bss are usually mapped) at greatly differing address ranges (> 16bit). This makes it necessary to support a large code model that uses absolute or GOT-relative addressing modes for these targets.

[llvmbot]

@llvm/issue-subscribers-backend-m68k

Author: Janis Heims (TechnoElf)

The currently supported code models all assume that any symbol may be reached with a 16bit offset, however, m68k-based microcontrollers may have ROM/flash (where .text and .rodata are usually mapped) and RAM (where .data and .bss are usually mapped) at greatly differing address ranges (> 16bit). This makes it necessary to support a large code model that uses absolute or GOT-relative addressing modes for these targets.