stddef: Work around an interoperability problem of tcc with glibc. Reported by Luca Saiu <positron@gnu.org> in <https://lists.gnu.org/archive/html/bug-gnulib/2021-03/msg00011.html>. * lib/stddef.in.h [__TINYC__]: In case of the special invocation convention, undefine the macros that TinyCC's <stddef.h> should undefine but doesn't. * doc/posix-headers/stddef.texi: Mention the TinyCC bug.