`alignas` on C23

In the latest MacOS CI jobs there's the following failure:
```
Starting test 229/455 on Mon Sep  1 12:13:06 2025: testc11/alignas
alignas.c[6:0-0] : syntax error
Parsing errorFatal error: exception GoblintCil__Frontc.ParseError("Parse error")
make: *** [testc11/alignas] Error 2
```
This seems to be because C23 replaces `alignas` macro with a keyword.