[libunwind] Implement handling for kRegisterUndefined for capability registers

[dstolfa]

This should make all the morello tests pass except for libunwind_get_registers_pass.cpp which fails with:

/home/ds815/cheri/llvm-project/libunwind/test/libunwind_get_registers.pass.cpp:104:2: error: "Test not implemented for this architecture" [-Werror,-W#warnings]
#warning "Test not implemented for this architecture"

The DWARF that was problematic looked roughly like this:

  DW_CFA_undefined: reg228
  DW_CFA_advance_loc: 28
  DW_CFA_def_cfa: reg227 +32
  DW_CFA_offset_extended: reg228 -16
  DW_CFA_offset_extended: reg227 -32

[arichardson]

This matches getSavedRegister() so seems completely reasonable. Just one minor suggestion

[jrtc27]

Oh, actually, you don't need the define at all. Both uintcap_t and fake_capability have default initialisers that'll give you 0, so return fake_capability_t(); should be all you need.