[gdb/symtab] Fix data race on per_cu->dwarf_version

[gdb/symtab] Fix data race on per_cu->dwarf_version

When building gdb with -fsanitize=thread and gcc 12, and running test-case
gdb.dwarf2/dwz.exp, we run into a data race between thread T2 and the main
thread in the same write:
...
Write of size 1 at 0x7b200000300c:^M
    #0 cutu_reader::cutu_reader(dwarf2_per_cu_data*, dwarf2_per_objfile*, \
    abbrev_table*, dwarf2_cu*, bool, abbrev_cache*) gdb/dwarf2/read.c:6252 \
    (gdb+0x82f3b3)^M
...
which is here:
...
         this_cu->dwarf_version = cu->header.version;
...

Both writes are called from the parallel for in dwarf2_build_psymtabs_hard,
this one directly:
...
    #1 process_psymtab_comp_unit gdb/dwarf2/read.c:6774 (gdb+0x8304d7)^M
    #2 operator() gdb/dwarf2/read.c:7098 (gdb+0x8317be)^M
    #3 operator() gdbsupport/parallel-for.h:163 (gdb+0x872380)^M
...
and this via the PU import:
...
    #1 cooked_indexer::ensure_cu_exists(cutu_reader*, dwarf2_per_objfile*, \
    sect_offset, bool,  bool) gdb/dwarf2/read.c:17964 (gdb+0x85c43b)^M
    #2 cooked_indexer::index_imported_unit(cutu_reader*, unsigned char const*, \
    abbrev_info const*) gdb/dwarf2/read.c:18248 (gdb+0x85d8ff)^M
    #3 cooked_indexer::index_dies(cutu_reader*, unsigned char const*, \
    cooked_index_entry const*, bool) gdb/dwarf2/read.c:18302 (gdb+0x85dcdb)^M
    #4 cooked_indexer::make_index(cutu_reader*) gdb/dwarf2/read.c:18443 \
    (gdb+0x85e68a)^M
    #5 process_psymtab_comp_unit gdb/dwarf2/read.c:6812 (gdb+0x830879)^M
    #6 operator() gdb/dwarf2/read.c:7098 (gdb+0x8317be)^M
    #7 operator() gdbsupport/parallel-for.h:171 (gdb+0x8723e2)^M
...

Fix this by setting the field earlier, in read_comp_units_from_section.

The write in cutu_reader::cutu_reader() is still needed, in case
read_comp_units_from_section is not used (run the test-case with say, target
board cc-with-gdb-index).

Make the write conditional, such that it doesn't trigger if the field is
already set by read_comp_units_from_section.  Instead, verify that the
field already has the value that we're trying to set it to.

Move this logic into into a member function set_version (in analogy to the
already present member function version) to make sure it's used consistenly,
and make the field private in order to enforce access through the member
functions, and rename it to m_dwarf_version.

While we're at it, make sure that the version is set before read, to avoid
say returning true for "per_cu.version () < 5" if "per_cu.version () == 0".

Tested on x86_64-linux.