C++ strict mode drops the GNU ", ##__VA_ARGS__" comma elision, so
GET_ARG_COUNT() and REVERSE_ARG() misselect their dispatch entry with
zero varargs. Centralize the empty-argument handling in GET_ARG_COUNT
(via __VA_OPT__ for C++) and make REVERSE_ARG and FOREACH_ARG dispatch
through CONCATENATE(prefix, GET_ARG_COUNT(...)), removing the two
duplicated 33-entry selector lists.
Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>
