libunwind compile error on x32 using clang/clang++

When built with clang libunwind fails with:
```
/mnt/vmtmp/una/repo/llvm/libunwind/src/DwarfParser.hpp:480:8: error: non-constant-expression cannot be narrowed from type 'link_reg_t' (aka 'unsigned long long') to 'pint_t' (aka 'unsigned int') in initializer list [-Wc++11-narrowing]
480 | upToPC - fdeInfo.pcStart}};
| ^~~~~~~~~~~~~~~~~~~~~~~~
/mnt/vmtmp/una/repo/llvm/libunwind/src/DwarfInstructions.hpp:220:33: note: in instantiation of function template specialization 'libunwind::CFI_Parser::parseFDEInstructions' requested here
220 | if (CFI_Parser::template parseFDEInstructions(
| ^
/mnt/vmtmp/una/repo/llvm/libunwind/src/UnwindCursor.hpp:1079:37: note: in instantiation of member function 'libunwind::DwarfInstructions::stepWithDwarf' requested here
1079 | return DwarfInstructions::stepWithDwarf(
| ^
/mnt/vmtmp/una/repo/llvm/libunwind/src/UnwindCursor.hpp:3279:20: note: in instantiation of member function 'libunwind::UnwindCursor::stepWithDwarfFDE' requested here
3279 | result = this->stepWithDwarfFDE(stage2);
| ^
/mnt/vmtmp/una/repo/llvm/libunwind/src/UnwindCursor.hpp:1384:21: note: in instantiation of member function 'libunwind::UnwindCursor::step' requested here
1384 | UnwindCursor::UnwindCursor(unw_context_t *context, A &as)
| ^
/mnt/vmtmp/una/repo/llvm/libunwind/src/libunwind.cpp:87:7: note: in instantiation of member function 'libunwind::UnwindCursor::UnwindCursor' requested here
87 | UnwindCursor(
| ^
/mnt/vmtmp/una/repo/llvm/libunwind/src/DwarfParser.hpp:480:8: note: insert an explicit cast to silence this issue
480 | upToPC - fdeInfo.pcStart}};
| ^~~~~~~~~~~~~~~~~~~~~~~~
| static_cast( )
In file included from /mnt/vmtmp/una/repo/llvm/libunwind/src/libunwind.cpp:30:
/mnt/vmtmp/una/repo/llvm/libunwind/src/AddressSpace.hpp:669:46: error: non-constant-expression cannot be narrowed from type 'link_reg_t' (aka 'unsigned long long') to 'uintptr_t' (aka 'unsigned int') in initializer list [-Wc++11-narrowing]
669 | dl_iterate_cb_data cb_data = {this, &info, targetAddr};
| ^~~~~~~~~~
/mnt/vmtmp/una/repo/llvm/libunwind/src/UnwindCursor.hpp:2779:30: note: in instantiation of function template specialization 'libunwind::LocalAddressSpace::findUnwindSections' requested here
2779 | if (_addressSpace.template findUnwindSections(pc, sects)) {
| ^
/mnt/vmtmp/una/repo/llvm/libunwind/src/UnwindCursor.hpp:1384:21: note: in instantiation of member function 'libunwind::UnwindCursor::setInfoBasedOnIPRegister' requested here
1384 | UnwindCursor::UnwindCursor(unw_context_t *context, A &as)
| ^
/mnt/vmtmp/una/repo/llvm/libunwind/src/libunwind.cpp:87:7: note: in instantiation of member function 'libunwind::UnwindCursor::UnwindCursor' requested here
87 | UnwindCursor(
| ^
/mnt/vmtmp/una/repo/llvm/libunwind/src/AddressSpace.hpp:669:46: note: insert an explicit cast to silence this issue
669 | dl_iterate_cb_data cb_data = {this, &info, targetAddr};
| ^~~~~~~~~~
| static_cast( )
2 errors generated.

Reproduce the x32 clang/clang++ build with `una/una.py --int --build`. Start with the reported initializers in `libunwind/src/DwarfParser.hpp` and `libunwind/src/AddressSpace.hpp`, then inspect the related instantiations in `DwarfInstructions.hpp`, `UnwindCursor.hpp`, and `libunwind.cpp`. Done means libunwind builds successfully for x32 with clang without the reported narrowing errors.

Written by the indexing model from the issue text.