Mark `sigma` and `tau` as as `__non_string`, to avoid warnings from
clang 21 similar to:
sys/crypto/chacha20/chacha.c:53:31: error: initializer-string for character array is too long, array size is 16 but initializer has size 17 (including the null terminating character); did you mean to use the 'nonstring' attribute? [-Werror,-Wunterminated-string-initialization]
53 | static const char sigma[16] = "expand 32-byte k";
| ^~~~~~~~~~~~~~~~~~
sys/crypto/chacha20/chacha.c:54:29: error: initializer-string for character array is too long, array size is 16 but initializer has size 17 (including the null terminating character); did you mean to use the 'nonstring' attribute? [-Werror,-Wunterminated-string-initialization]
54 | static const char tau[16] = "expand 16-byte k";
| ^~~~~~~~~~~~~~~~~~
MFC after: 3 days