Add some new DW_IDX_* constants

I've reimplemented the .debug_names code in GDB -- it was quite far
from being correct, and the new implementation is much closer to what
is specified by DWARF.

However, the new writer in GDB needs to emit some symbol properties,
so that the reader can be fully functional.  This patch adds a few new
DW_IDX_* constants, and tries to document the existing extensions as
well.  (My patch series add more documentation of these to the GDB
manual as well.)

include/dwarf2.def

@@ -802,8 +802,17 @@ DW_IDX (DW_IDX_parent, 4)
 DW_IDX (DW_IDX_type_hash, 5)
 DW_IDX_DUP (DW_IDX_lo_user, 0x2000)
 DW_IDX (DW_IDX_hi_user, 0x3fff)
+/* Internal linkage.  A flag.  */
 DW_IDX (DW_IDX_GNU_internal, 0x2000)
+/* External linkage.  A flag.  Note that gdb no longer generates this;
+   the default is to assume external linkage.  */
 DW_IDX (DW_IDX_GNU_external, 0x2001)
+/* This entry is the program's entry point.  A flag.  */
+DW_IDX (DW_IDX_GNU_main, 0x2002)
+/* Language for this entry.  A DW_LANG_* value.  */
+DW_IDX (DW_IDX_GNU_language, 0x2003)
+/* This entry is a linkage name.  A flag.  */
+DW_IDX (DW_IDX_GNU_linkage_name, 0x2004)
 DW_END_IDX
 
 /* DWARF5 Unit type header encodings  */