[gdb/symtab] Fix data race on objfile->template_symbols

Data race between:
...
  Read of size 8 at 0x7b4000006dd8 by thread T4:
    #0 new_symbol gdb/dwarf2/read.c:21085 (gdb+0x868212)
    #1 handle_struct_member_die gdb/dwarf2/read.c:14828 (gdb+0x851be8)
    #2 process_structure_scope gdb/dwarf2/read.c:14865 (gdb+0x851e70)
    #3 process_die gdb/dwarf2/read.c:8649 (gdb+0x83a048)
    #4 read_namespace gdb/dwarf2/read.c:16026 (gdb+0x857a32)
    #5 process_die gdb/dwarf2/read.c:8689 (gdb+0x83a100)
    #6 read_file_scope gdb/dwarf2/read.c:9616 (gdb+0x83cb81)
    #7 process_die gdb/dwarf2/read.c:8620 (gdb+0x839f43)
    #8 process_full_comp_unit gdb/dwarf2/read.c:8383 (gdb+0x839509)
    #9 process_queue_item gdb/dwarf2/read.c:7592 (gdb+0x8359f5)
...
and:
...
  Previous write of size 8 at 0x7b4000006dd8 by main thread:
    #0 new_symbol gdb/dwarf2/read.c:21086 (gdb+0x868247)
    #1 handle_struct_member_die gdb/dwarf2/read.c:14828 (gdb+0x851be8)
    #2 process_structure_scope gdb/dwarf2/read.c:14865 (gdb+0x851e70)
    #3 process_die gdb/dwarf2/read.c:8649 (gdb+0x83a048)
    #4 read_namespace gdb/dwarf2/read.c:16026 (gdb+0x857a32)
    #5 process_die gdb/dwarf2/read.c:8689 (gdb+0x83a100)
    #6 read_file_scope gdb/dwarf2/read.c:9616 (gdb+0x83cb81)
    #7 process_die gdb/dwarf2/read.c:8620 (gdb+0x839f43)
    #8 process_full_comp_unit gdb/dwarf2/read.c:8383 (gdb+0x839509)
    #9 process_queue_item gdb/dwarf2/read.c:7592 (gdb+0x8359f5)
...

Fix this by adding a lock in new_symbol for adding to
objfile->template_symbols.

gdb/dwarf2/read.c

@@ -21098,6 +21098,10 @@ new_symbol (struct die_info *die, struct type *type, struct dwarf2_cu *cu,
 
       if (suppress_add)
 	{
+#if CXX_STD_THREAD
+	  static std::mutex template_symbols_lock;
+	  std::lock_guard<std::mutex> guard (template_symbols_lock);
+#endif
 	  sym->hash_next = objfile->template_symbols;
 	  objfile->template_symbols = sym;
 	  list_to_add = NULL;