Suppress MemorySanitizer false positive

MemorySanitizer does not support assembly, and therefore produces a
false positive on `blake3_hasher_finalize` and related functions.

c/blake3.c

@@ -5,6 +5,13 @@
 #include "blake3.h"
 #include "blake3_impl.h"
 
+#ifdef __has_feature
+#if __has_feature(memory_sanitizer)
+#include <sanitizer/msan_interface.h>
+#define BLAKE3_MEMORY_SANITIZER_BUILD
+#endif
+#endif
+
 const char *blake3_version(void) { return BLAKE3_VERSION_STRING; }
 
 INLINE void chunk_state_init(blake3_chunk_state *self, const uint32_t key[8],
@@ -107,6 +114,10 @@ INLINE void output_root_bytes(const output_t *self, uint64_t seek, uint8_t *out,
     output_block_counter += 1;
     offset_within_block = 0;
   }
+#ifdef BLAKE3_MEMORY_SANITIZER_BUILD
+  // MemorySanitizer gives a false positive due to use of assembly.
+  __msan_unpoison(out, out_len);
+#endif
 }
 
 INLINE void chunk_state_update(blake3_chunk_state *self, const uint8_t *input,