rb_str_hash(): Avoid UB with making misaligned pointer

Previously, on common platforms, this code made a pointer to a union of
8 byte alignment out of a char pointer that is not guaranteed to satisfy
the alignment requirement. That is undefined behavior according
to [C99 6.3.2.3p7](https://port70.net/~nsz/c/c99/n1256.html#6.3.2.3p7).

Use memcpy() to do the unaligned read instead.

string.c

@@ -3735,8 +3735,8 @@ st_index_t
 rb_str_hash(VALUE str)
 {
     if (FL_TEST_RAW(str, STR_PRECOMPUTED_HASH)) {
-        typedef struct {char bytes[sizeof(st_index_t)];} unaligned_index;
-        st_index_t precomputed_hash = ((union {st_index_t i; unaligned_index b;} *)(RSTRING_END(str) + TERM_LEN(str)))->i;
+        st_index_t precomputed_hash;
+        memcpy(&precomputed_hash, RSTRING_END(str) + TERM_LEN(str), sizeof(precomputed_hash));
 
         RUBY_ASSERT(precomputed_hash == str_do_hash(str));
         return precomputed_hash;