Fix "base" calculation for rangelists.

This fixes finding inline functions in DWARF5.

1) if there's no base in the original DIE, use the one from the root of
   the TU:

   ```The applicable base address of a range list entry is determined by
   the closest preceding base address selection entry (see below) in the
   same range list. If there is no such selection entry, then the
   applicable base address defaults to the base address of the
   compilation unit```

2) Fix base calculations for DWARFv5 which was using a local variable
   for the 'base' rather than the passed value.

dwarf_die.cc

@@ -488,7 +488,6 @@ Ranges::Ranges(const DIE &die, uintmax_t base) {
         // const auto &elf = die.getUnit()->dwarf->elf;
         // auto &addrs = elf->getDebugSection(".debug_addr", SHT_NULL); // XXX: would be used by the "x" ops below
 
-        uintmax_t base = 0;
         auto addrlen = die.getUnit()->addrlen;
         auto &unit = *die.getUnit();
         for (bool done = false; !done;) {

dwarf_unit.cc

@@ -211,6 +211,12 @@ Unit::findAbbreviation(size_t code) const
 
 const std::unique_ptr<Ranges> &
 Unit::getRanges(const DIE &die, uintmax_t base) {
+    if (base == 0) {
+       const DIE::Attribute & low = root().attribute(DW_AT_low_pc);
+       if (low.valid())
+          base = uintmax_t(low);
+
+    }
     auto &ptr = rangesForOffset[die.offset];
     if (ptr == nullptr)
         ptr = std::make_unique<Ranges>(die, base);