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.

1 files changed, 23 insertions, 6 deletions

diff --git a/gdb/elfread.c b/gdb/elfread.c
index 31288a9..7a41c26 100644
--- a/gdb/elfread.c
+++ b/gdb/elfread.c
@@ -48,6 +48,11 @@
 #include "location.h"
 #include "auxv.h"
 
+/* Forward declarations.  */
+extern const struct sym_fns elf_sym_fns_gdb_index;
+extern const struct sym_fns elf_sym_fns_debug_names;
+extern const struct sym_fns elf_sym_fns_lazy_psyms;
+
 /* The struct elfinfo is available only during ELF symbol table and
    psymtab reading.  It is destroyed at the completion of psymtab-reading.
    It's local to elf_symfile_read.  */
@@ -1211,11 +1216,25 @@ elf_symfile_read (struct objfile *objfile, symfile_add_flags symfile_flags)
 
   if (dwarf2_has_info (objfile, NULL))
     {
-      /* elf_sym_fns_gdb_index cannot handle simultaneous non-DWARF debug
-	 information present in OBJFILE.  If there is such debug info present
-	 never use .gdb_index.  */
+      dw_index_kind index_kind;
 
-      if (objfile_has_partial_symbols (objfile))
+      /* elf_sym_fns_gdb_index cannot handle simultaneous non-DWARF
+	 debug information present in OBJFILE.  If there is such debug
+	 info present never use an index.  */
+      if (!objfile_has_partial_symbols (objfile)
+	  && dwarf2_initialize_objfile (objfile, &index_kind))
+	{
+	  switch (index_kind)
+	    {
+	    case dw_index_kind::GDB_INDEX:
+	      objfile_set_sym_fns (objfile, &elf_sym_fns_gdb_index);
+	      break;
+	    case dw_index_kind::DEBUG_NAMES:
+	      objfile_set_sym_fns (objfile, &elf_sym_fns_debug_names);
+	      break;
+	    }
+	}
+      else
 	{
 	  /* It is ok to do this even if the stabs reader made some
 	     partial symbols, because OBJF_PSYMTABS_READ has not been
@@ -1223,8 +1242,6 @@ elf_symfile_read (struct objfile *objfile, symfile_add_flags symfile_flags)
 	     when needed.  */
 	  objfile_set_sym_fns (objfile, &elf_sym_fns_lazy_psyms);
 	}
-      else
-	objfile_set_sym_fns (objfile, &dwarf2_initialize_objfile (objfile));
     }
   /* If the file has its own symbol tables it has no separate debug
      info.  `.dynsym'/`.symtab' go to MSYMBOLS, `.debug_info' goes to