Rewrite .debug_names writer

This rewrites GDB's .debug_names writer.  It is now closer to the form
imagined in the DWARF spec.  In particular, names are emitted exactly
as they appear in the original DWARF.

In order to make the reader work nicely, some extensions were needed.
These were all documented in an earlier patch.  Note that in
particular this writer solves the "main name" problem by putting a
flag into the table.

GDB does not use the .debug_names hash table, so it also does not
write one.  I consider this hash table to be essentially useless in
general, due to the name canonicalization problem -- while DWARF says
that writers should use the system demangling style, (1) this style
varies across systems, so it can't truly be relied on; and (2) at
least GCC and one other compiler don't actually follow this part of
the spec anyway.

It's important to note, though, that even if the hash was somehow
useful, GDB probably still would not use it -- a sorted list of names
is needed for completion and performs reasonably well for other
lookups, so a hash table is just overhead, IMO.

String emission is also simplified.  There's no need in this writer to
ingest the contents of .debug_str.

A couple of tests are updated to reflect the fact that they now "fail"
because the tests don't include .debug_aranges in the .S file.
Arguably the .debug_names writer should also create this section; but
I did not implement that in this series, and there is a separate bug
about it.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=24820
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=24549