S390: Fix building with --disable-mutli-arch [BZ #31196]

Starting with commits
- 7ea51012
string: Add libc_hidden_proto for strchrnul
- 22999b2f
string: Add libc_hidden_proto for memrchr

building glibc on s390x with --disable-multi-arch fails if only
the C-variant of strchrnul / memrchr is used.  This is the case
if gcc uses -march < z13.

The build fails with:
../sysdeps/s390/strchrnul-c.c:28:49: error: ‘__strchrnul_c’ undeclared here (not in a function); did you mean ‘__strchrnul’?
   28 | __hidden_ver1 (__strchrnul_c, __GI___strchrnul, __strchrnul_c);

With --disable-multi-arch, __strchrnul_c is not available as string/strchrnul.c
is just included without defining STRCHRNUL and thus we also don't have to create
the internal hidden symbol.

Tested-by: Andreas K. Hüttel <dilfridge@gentoo.org>