Ignore 0,0 entries in .debug_aranges

When running the internal AdaCore test suite against the new DWARF
indexer, I found one regression on RISC-V.  The test in question uses
--gc-sections, and winds up with an entry in the middle of a
.debug_aranges that has both address and length of 0.  In this
scenario, gdb assumes the entries are terminated and then proceeds to
reject the section because it reads a subsequent entry as if it were a
header.

It seems to me that, because each header describes the size of each
.debug_aranges CU, it's better to simply ignore 0,0 entries and simply
read to the end.  That is what this patch does.

I've patched an existing test to provide a regression test for this.

gdb/dwarf2/read.c

@@ -2469,7 +2469,7 @@ read_addrmap_from_aranges (dwarf2_per_objfile *per_objfile,
 	 bytes.  */
       addr += (entry_end - addr) % (2 * address_size);
 
-      for (;;)
+      while (addr < entry_end)
 	{
 	  if (addr + 2 * address_size > entry_end)
 	    {
@@ -2487,7 +2487,14 @@ read_addrmap_from_aranges (dwarf2_per_objfile *per_objfile,
 						      dwarf5_byte_order);
 	  addr += address_size;
 	  if (start == 0 && length == 0)
-	    break;
+	    {
+	      /* This can happen on some targets with --gc-sections.
+		 This pair of values is also used to mark the end of
+		 the entries for a given CU, but we ignore it and
+		 instead handle termination using the check at the top
+		 of the loop.  */
+	      continue;
+	    }
 	  if (start == 0 && !per_bfd->has_section_at_zero)
 	    {
 	      /* Symbol was eliminated due to a COMDAT group.  */

gdb/testsuite/gdb.dwarf2/locexpr-data-member-location.exp

@@ -296,6 +296,9 @@ Dwarf::assemble ${asm_file} {
     }
 
     aranges {} cu_label {
+	# This 0,0 entry tests that the .debug_aranges reader can
+	# handle an apparent terminator before the end of the ranges.
+	arange {} 0 0
 	arange {} $foo_start $foo_end
 	arange {} $bar_start $bar_end
     }