[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__
```
