S390: Influence hwcaps/stfle via GLIBC_TUNABLES.

This patch enables the option to influence hwcaps and stfle bits used
by the s390 specific ifunc-resolvers.  The currently x86-specific
tunable glibc.cpu.hwcaps is also used on s390x to achieve the task. In
addition the user can also set a CPU arch-level like z13 instead of
single HWCAP and STFLE features.

Note that the tunable only handles the features which are really used
in the IFUNC-resolvers.  All others are ignored as the values are only
used inside glibc.  Thus we can influence:
- HWCAP_S390_VXRS (z13)
- HWCAP_S390_VXRS_EXT (z14)
- HWCAP_S390_VXRS_EXT2 (z15)
- STFLE_MIE3 (z15)

The influenced hwcap/stfle-bits are stored in the s390-specific
cpu_features struct which also contains reserved fields for future
usage.

The ifunc-resolvers and users of stfle bits are adjusted to use the
information from cpu_features struct.

On 31bit, the ELF_MACHINE_IRELATIVE macro is now also defined.
Otherwise the new ifunc-resolvers segfaults as they depend on
the not yet processed_rtld_global_ro@GLIBC_PRIVATE relocation.

sysdeps/s390/multiarch/ifunc-impl-list.c

@@ -19,6 +19,7 @@
 #include <assert.h>
 #include <string.h>
 #include <wchar.h>
+#include <cpu-features.h>
 #include <ifunc-impl-list.h>
 #include <ifunc-resolve.h>
 #include <ifunc-memset.h>
@@ -78,14 +79,10 @@ __libc_ifunc_impl_list (const char *name, struct libc_ifunc_impl *array,
   size_t i __attribute__ ((unused)) = max;
 
   /* Get hardware information.  */
-  unsigned long int dl_hwcap = GLRO (dl_hwcap);
-  unsigned long long stfle_bits = 0ULL;
-  if ((dl_hwcap & HWCAP_S390_STFLE)
-	&& (dl_hwcap & HWCAP_S390_ZARCH)
-	&& (dl_hwcap & HWCAP_S390_HIGH_GPRS))
-    {
-      S390_STORE_STFLE (stfle_bits);
-    }
+  const struct cpu_features *features = &GLRO(dl_s390_cpu_features);
+  unsigned long int dl_hwcap = features->hwcap;
+  const unsigned long long * __attribute__((unused)) stfle_bits
+    = features->stfle_bits;
 
 #if HAVE_MEMSET_IFUNC
   IFUNC_IMPL (i, name, memset,