Cgo add cbytes implementation (rebased version of #3318) (#4470)

cgo: added CBytes implementation

src/runtime/string.go

@@ -283,3 +283,10 @@ func cgo_GoBytes(ptr unsafe.Pointer, length uintptr) []byte {
 	}
 	return buf
 }
+
+func cgo_CBytes(b []byte) unsafe.Pointer {
+	p := malloc(uintptr(len(b)))
+	s := unsafe.Slice((*byte)(p), len(b))
+	copy(s, b)
+	return p
+}