RARRAY_EMBED_LEN/RSTRING_EMBED_LEN: add RUBY3_CAST

RUBY3_CAST is a macro to suppress g++/clang++ warnings about C-style
casts.  Though Ruby core don't have to bother C++ situations, extension
libraries can benefit from this.

include/ruby/3/core/rarray.h

@@ -123,7 +123,7 @@ RARRAY_EMBED_LEN(VALUE ary)
     VALUE f = RBASIC(ary)->flags;
     f &= RARRAY_EMBED_LEN_MASK;
     f >>= RARRAY_EMBED_LEN_SHIFT;
-    return (long)f;
+    return RUBY3_CAST((long)f);
 }
 
 RUBY3_ATTR_PURE_ON_NDEBUG()

include/ruby/3/core/rstring.h

@@ -108,7 +108,7 @@ RSTRING_EMBED_LEN(VALUE str)
     VALUE f = RBASIC(str)->flags;
     f &= RSTRING_EMBED_LEN_MASK;
     f >>= RSTRING_EMBED_LEN_SHIFT;
-    return (long)f;
+    return RUBY3_CAST((long)f);
 }
 
 RUBY3_ATTR_PURE_ON_NDEBUG()