[gdb/symtab] Fix data race in objstats->n_syms

[gdb/symtab] Fix data race in objstats->n_syms

Data race between:
...
  Read of size 4 at 0x7b4000006dc8 by main thread:
    #0 new_symbol gdb/dwarf2/read.c:20704 (gdb+0x866f3e)
    #1 process_die gdb/dwarf2/read.c:8674 (gdb+0x839fa8)
    #2 read_file_scope gdb/dwarf2/read.c:9610 (gdb+0x83ca7f)
    #3 process_die gdb/dwarf2/read.c:8614 (gdb+0x839e41)
    #4 process_full_comp_unit gdb/dwarf2/read.c:8383 (gdb+0x839480)
    #5 process_queue_item gdb/dwarf2/read.c:7592 (gdb+0x83599f)
...
and:
...
  Previous write of size 4 at 0x7b4000006dc8 by thread T2:
    #0 new_symbol gdb/dwarf2/read.c:20704 (gdb+0x866f5d)
    #1 process_die gdb/dwarf2/read.c:8674 (gdb+0x839fa8)
    #2 read_file_scope gdb/dwarf2/read.c:9610 (gdb+0x83ca7f)
    #3 process_die gdb/dwarf2/read.c:8614 (gdb+0x839e41)
    #4 process_full_comp_unit gdb/dwarf2/read.c:8383 (gdb+0x839480)
    #5 process_queue_item gdb/dwarf2/read.c:7592 (gdb+0x83599f)
...

Fix by making objstats->n_syms atomic.  Likewise for n_types.