s390x: Require GCC 7.1 or later to build glibc.

GCC 6.5 fails to correctly build ldconfig with recent ld.so.cache
commits, e.g.:
785969a047
elf: Implement a string table for ldconfig, with tail merging

If glibc is build with gcc 6.5.0:
__builtin_add_overflow is used in
<glibc>/elf/stringtable.c:stringtable_finalize()
which leads to ldconfig failing with "String table is too large".
This is also recognizable in following tests:
FAIL: elf/tst-glibc-hwcaps-cache
FAIL: elf/tst-glibc-hwcaps-prepend-cache
FAIL: elf/tst-ldconfig-X
FAIL: elf/tst-ldconfig-bad-aux-cache
FAIL: elf/tst-ldconfig-ld_so_conf-update
FAIL: elf/tst-stringtable

See gcc "Bug 98269 - gcc 6.5.0 __builtin_add_overflow() with small
uint32_t values incorrectly detects overflow"
(https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98269)

INSTALL

@@ -467,6 +467,9 @@ build the GNU C Library:
 
      For ARC architecture builds, GCC 8.3 or higher is needed.
 
+     For s390x architecture builds, GCC 7.1 or higher is needed (See gcc
+     Bug 98269).
+
      For multi-arch support it is recommended to use a GCC which has
      been built with support for GNU indirect functions.  This ensures
      that correct debugging information is generated for functions

NEWS

@@ -67,6 +67,8 @@ Changes to build and runtime requirements:
   (and pseudo-terminals are supported), a devpts file system is mounted
   on /dev/pts.  Current systems already meet these requirements.
 
+* s390x requires GCC 7.1 or newer.  See gcc Bug 98269.
+
 Security related changes:
 
   CVE-2020-27618: An infinite loop has been fixed in the iconv program when

manual/install.texi

@@ -509,6 +509,8 @@ Library with support for IEEE long double.
 
 For ARC architecture builds, GCC 8.3 or higher is needed.
 
+For s390x architecture builds, GCC 7.1 or higher is needed (See gcc Bug 98269).
+
 For multi-arch support it is recommended to use a GCC which has been built with
 support for GNU indirect functions.  This ensures that correct debugging
 information is generated for functions selected by IFUNC resolvers.  This

sysdeps/s390/configure

@@ -296,5 +296,40 @@ then
 
 fi
 
+
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking if $CC is sufficient to build libc on s390x" >&5
+$as_echo_n "checking if $CC is sufficient to build libc on s390x... " >&6; }
+if ${libc_cv_compiler_ok_on_s390x+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+
+cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+int
+main ()
+{
+
+#if !defined __GNUC__ || __GNUC__ < 7 || (__GNUC__ == 7 && __GNUC_MINOR__ < 1)
+#error insufficient compiler for building on s390x
+#endif
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+  libc_cv_compiler_ok_on_s390x=yes
+else
+  libc_cv_compiler_ok_on_s390x=no
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $libc_cv_compiler_ok_on_s390x" >&5
+$as_echo "$libc_cv_compiler_ok_on_s390x" >&6; }
+if test "$libc_cv_compiler_ok_on_s390x" != yes; then
+   critic_missing="$critic_missing On s390x, GCC >= 7.1.0 is required."
+fi
+
 test -n "$critic_missing" && as_fn_error $? "
 *** $critic_missing" "$LINENO" 5

sysdeps/s390/configure.ac

@@ -212,5 +212,22 @@ then
   AC_DEFINE(HAVE_S390_MIN_ARCH13_ZARCH_ASM_SUPPORT)
 fi
 
+dnl test if GCC is new enough. See gcc "Bug 98269 - gcc 6.5.0
+dnl __builtin_add_overflow() with small uint32_t values incorrectly detects
+dnl overflow
+dnl (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98269)
+AC_CACHE_CHECK([if $CC is sufficient to build libc on s390x],
+libc_cv_compiler_ok_on_s390x, [
+AC_TRY_COMPILE([], [
+#if !defined __GNUC__ || __GNUC__ < 7 || (__GNUC__ == 7 && __GNUC_MINOR__ < 1)
+#error insufficient compiler for building on s390x
+#endif
+],
+	[libc_cv_compiler_ok_on_s390x=yes],
+	[libc_cv_compiler_ok_on_s390x=no])])
+if test "$libc_cv_compiler_ok_on_s390x" != yes; then
+   critic_missing="$critic_missing On s390x, GCC >= 7.1.0 is required."
+fi
+
 test -n "$critic_missing" && AC_MSG_ERROR([
 *** $critic_missing])