Unbreak build for non-ELF ports

Unbreak build for non-ELF ports

As reported at
<https://sourceware.org/ml/gdb-patches/2017-12/msg00229.html>, this
commit:

~~~~
 commit abccd1e7b7a37385159610ca4e0bc2632a547e9a
 Author:     Jan Kratochvil <jan.kratochvil@redhat.com>
 AuthorDate: Fri Dec 8 22:44:11 2017 +0000

     Change dwarf2_initialize_objfile's return value

     dwarf2_initialize_objfile was returning boolean whether it is psymtabs
     or .gdb_index while now it needs to return also whether it is
     .debug_names.
~~~~

breaks non-ELF-target builds:

 dwarf2read.o: In function `dwarf2_initialize_objfile(objfile*)':
 /home/yao.qi/SourceCode/gnu/binutils-gdb/gdb/dwarf2read.c:6486:
 undefined reference to `elf_sym_fns_gdb_index'
 /home/yao.qi/SourceCode/gnu/binutils-gdb/gdb/dwarf2read.c:6490:
 undefined reference to `elf_sym_fns_debug_names'
 /home/yao.qi/SourceCode/gnu/binutils-gdb/gdb/dwarf2read.c:6495:
 undefined reference to `elf_sym_fns_lazy_psyms'
 collect2: error: ld returned 1 exit status
	 Makefile:1920: recipe for target 'gdb' failed

because gdb/elfread.c is not included in the gdb build unless bfd also
includes elf support.

Fix this by reverting the patch mentioned above and at the same time
re-adding .debug_names support by adding a new output parameter to
dwarf2_initialize_objfile to indicate the index variant in use.  We
can reuse the new dw_index_kind enum in dwarf2read.c for that.

gdb/ChangeLog:
2017-12-11  Pedro Alves  <palves@redhat.com>

	* defs.h (elf_sym_fns_lazy_psyms, elf_sym_fns_gdb_index)
	(elf_sym_fns_debug_names): Move to elfread.c.
	* dwarf2read.c (dwarf2_initialize_objfile): Return a boolean
	instead of a sym_fns and add 'index_kind' output parameter.  Fill
	the latter in with the index variant kind if using an index.
	(enum dw_index_kind): Moved to symfile.h.
	* elfread.c (elf_sym_fns_gdb_index, elf_sym_fns_debug_names)
	(elf_sym_fns_lazy_psyms): Move from defs.h.
	(elf_symfile_read): Adjust to new dwarf2_initialize_objfile
	interface.
	* symfile.h (enum class dw_index_kind): New, moved from
	dwarf2read.c.
	(dwarf2_initialize_objfile): Change prototype.