[gdb/symtab] Fix race condition in just_read_cus

[gdb/symtab] Fix race condition in just_read_cus

Race condition between:
...
  Read of size 8 at 0x7b640001f2c8 by thread T1:
    #0 std::vector<dwarf2_per_cu_data*, std::allocator<dwarf2_per_cu_data*>
    >::push_back(dwarf2_per_cu_data* const&)
    /usr/include/c++/12/bits/stl_vector.h:1278 (gdb+0x888ad6)
    #1 process_full_comp_unit gdb/dwarf2/read.c:8451 (gdb+0x8397d0)
    #2 process_queue_item gdb/dwarf2/read.c:7592 (gdb+0x8359f5)
...
and:
...
  Previous write of size 8 at 0x7b640001f2c8 by main thread:
    #0 std::vector<dwarf2_per_cu_data*, std::allocator<dwarf2_per_cu_data*>
    >::push_back(dwarf2_per_cu_data* const&)
    /usr/include/c++/12/bits/stl_vector.h:1283 (gdb+0x888b52)
    #1 process_full_comp_unit gdb/dwarf2/read.c:8451 (gdb+0x8397d0)
    #2 process_queue_item gdb/dwarf2/read.c:7592 (gdb+0x8359f5)
...

Fix this by doing the just_read_cus push_back in a lock.