[libunwind] Bare-metal DWARF: set dso_base to 0
Previously, DwarfFDECache::findFDE used 0 as a special value meaning
"search the entire cache, including dynamically-registered FDEs".
Switch this special value to -1, which doesn't make sense as a DSO
Reviewed By: compnerd, #libunwind
Differential Revision: https://reviews.llvm.org/D86748