Assign `-Wno-keyword-compat` to clang only

Assign `-Wno-keyword-compat` to clang only

The compiler flag `-Wno-keyword-compat` is not recognized by GCC, but by
clang. Although it does not harm the resulting compilation unit (GCC has a
special handling of `-Wno-*` flags), an additional warning message is issued if
a compilation unit fails or produces other warnings:

```
At global scope:
cc1plus: note: unrecognized command-line option '-Wno-keyword-compat' may have been intended to silence earlier diagnostics
```

This commit assigns `-Wno-keyword-compat` to clang only.

Signed-off-by: Simon Kuenzer <simon@unikraft.io>
Reviewed-by: Marco Schlumpp <marco@unikraft.io>
Reviewed-by: Stefan Jumarea <stefanjumarea02@gmail.com>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #35