crypto: avoid warnings about too-long initializer strings

[dim]

Summary

Mark sigma and tau 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

[dim]

Fix double word in commit message.