Added lfs_crc32c_cube, replacing lfsr_gcksum_cube

This is a bit on the edge of what is a reasonable to shove in
lfs_util.h. But it's useful enough that we were giving it its own
function anyways, and moving it to lfs_util.h allows it to be
potentially overriden with cube-specific optimizations (expanded
terms?).

No code changes.

lfs_util.h

@@ -635,6 +635,11 @@ uint32_t lfs_crc32c(uint32_t crc, const void *buffer, size_t size);
 // Multiply two crc32cs in the crc32c ring
 uint32_t lfs_crc32c_mul(uint32_t a, uint32_t b);
 
+// Find the cube of a crc32c in the crc32c ring
+static inline uint32_t lfs_crc32c_cube(uint32_t a) {
+    return lfs_crc32c_mul(lfs_crc32c_mul(a, a), a);
+}
+
 
 // Allocate memory, only used if buffers are not provided to littlefs
 // Note, memory must be 64-bit aligned