processor: suppress unused function warning for add_feature_mask_u32

The non-template base case is only called by template overloads used in
the old processor backends. The cpufeatures backend doesn't use these
templates, so clang -Werror,-Wunused-function flags it.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: gbaraldi

src/processor.cpp

@@ -169,7 +169,7 @@ struct FeatureList {
     }
 };
 
-static inline constexpr uint32_t add_feature_mask_u32(uint32_t mask, uint32_t u32idx)
+JL_UNUSED static inline constexpr uint32_t add_feature_mask_u32(uint32_t mask, uint32_t u32idx)
 {
     return mask;
 }