sparc: revive __HAVE_ARCH_STRLEN for 32bit sparc

Prior to commit 70a6fcf3 ("[sparc] unify 32bit and 64bit string.h"),
__HAVE_ARCH_STRLEN was defined in both of string_32.h and string_64.h

It did not unify __HAVE_ARCH_STRLEN, but deleted it from string_32.h

This issue was reported by the kbuild test robot in the trial of
forcible linking of $(lib-y) to vmlinux.

Fixes: 70a6fcf3 ("[sparc] unify 32bit and 64bit string.h")
Reported-by: kbuild test robot <lkp@intel.com>
Suggested-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Acked-by: David S. Miller <davem@davemloft.net>