configure.ac decides whether udiv_w_sdiv.c fallback is needed
based on $CHOST value (without $CFLAGS). longlong.h relies
on compiler macros (relies on $CFLAGS implicitly).

That leads to use of undefined macros. The workaround makes
udiv_w_sdiv.c a no-on for -mzarch.

https://bugs.gentoo.org/761310
--- a/mpn/generic/udiv_w_sdiv.c
+++ b/mpn/generic/udiv_w_sdiv.c
@@ -1,3 +1,4 @@
+#if !defined(__zarch__)
 /* mpn_udiv_w_sdiv -- implement udiv_qrnnd on machines with only signed
    division.
 
@@ -139,3 +140,4 @@ mpn_udiv_w_sdiv (mp_limb_t *rp, mp_limb_t a1, mp_limb_t a0, mp_limb_t d)
   *rp = r;
   return q;
 }
+#endif