[libc] Implement iswgraph entrypoint (#185339)

part of https://github.com/llvm/llvm-project/issues/185136;

This PR adds public entrypoints for the wide character classification
function iswgraph in LLVM libc, using the same pattern as the existing
iswalpha entrypoint.

using bellow cmd to test:

```shell
ninja libc.test.src.wctype.iswgraph_test.__unit__
```

GitOrigin-RevId: 4f76d16203a83627f8541385e1fe082ecd920e77