[ELF] Set vna_flags to VER_FLG_WEAK if all references are weak (#176673)

When all undefined references to a version are weak, set vna_flags to
VER_FLG_WEAK in the .gnu.version_r section. This enables glibc ld.so to
report a warning instead of an error when the required version is not
found at runtime, supporting optional dependencies.

Per https://sourceware.org/bugzilla/show_bug.cgi?id=24718#c20 ,
glibc rtld since 2.30 (BZ #24741) tolerates missing versioned symbols
when the runtime shared object defines the required version. With this
vna_flags VER_FLG_WEAK change, rtld can also tolerate a completely
missing version, printing a message like:

```
% LD_PRELOAD=c2.so ./a
./a: /tmp/t/v2/c2.so: weak version `v1' not found (required by /tmp/t/v2/b.so)
a
```

instead of a fatal error.

In addition, add a comment about `vn.vernaxus.size() == 1`.

GitOrigin-RevId: e34651fa557911b04d83a90fd20191358714c5e5