[gdb/build] Return const reference in target_read_auxv

In target_read_auxv we return a copy of an object:
...
gdb::optional<gdb::byte_vector>
target_read_auxv ()
{
  ...
  return info->data;
}
...

Return a const reference instead, saving a copy.

This is exposed by using std::pmr::polymorphic_allocator instead of
std::allocator in default_init_allocator.

Tested on x86_64-linux.