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

[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.