[gdb/build] Fix build with g++-4.8

[gdb/build] Fix build with g++-4.8

When building g++-4.8, we run into:
...
src/gdb/dwarf2/read.c:919:5: error: multiple fields in union \
  'partial_die_info::<anonymous union>' initialized
...

This is due to:
...
    union
    {
      struct
      {
       CORE_ADDR lowpc = 0;
       CORE_ADDR highpc = 0;
      };
      ULONGEST ranges_offset;
    };
...

The error looks incorrect, given that only one union member is initialized,
and does not reproduce with newer g++.

Nevertheless, work around this by moving the initialization to a constructor.

[ I considered just removing the initialization, with the idea that access
should be guarded by has_pc_info, but I ran into one failure in the testsuite,
for gdb.base/check-psymtab.exp due to add_partial_symbol using lowpc without
checking has_pc_info. ]

Tested on x86_64-linux.