libunwind: untested attempt to fix sparc64 build

sparc64 is the only architecture currently using the DWARF unwinder from
GCC 4.2.1. Old GCC and related libraries are being removed soon; absent
other changes sparc64 would be left with no unwinder when that happens.

Instead, commit these changes which should at least allow the LLVM
unwinder to build. Someone with access to the obolete sparc64 hardware
supported by FreeBSD will need to test the result.

PR: 233405
Submitted by: cem