Remove --enable-tunables configure option

And make always supported.  The configure option was added on glibc 2.25
and some features require it (such as hwcap mask, huge pages support, and
lock elisition tuning).  It also simplifies the build permutations.

Changes from v1:
 * Remove glibc.rtld.dynamic_sort changes, it is orthogonal and needs
   more discussion.
 * Cleanup more code.
Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>

elf/tst-env-setuid-tunables.c

@@ -19,8 +19,6 @@
    glibc.malloc.check and glibc.malloc.mmap_threshold but also retain
    glibc.malloc.mmap_threshold in an unprivileged child.  */
 
-/* This is compiled as part of the testsuite but needs to see
-   HAVE_TUNABLES. */
 #define _LIBC 1
 #include "config.h"
 #undef _LIBC
@@ -83,7 +81,6 @@ test_child (int off)
 {
   const char *val = getenv ("GLIBC_TUNABLES");
 
-#if HAVE_TUNABLES
   if (val != NULL && strcmp (val, resultstrings[off]) == 0)
     return 0;
 
@@ -91,14 +88,6 @@ test_child (int off)
     printf ("[%d] Unexpected GLIBC_TUNABLES VALUE %s\n", off, val);
 
   return 1;
-#else
-  if (val != NULL)
-    {
-      printf ("[%d] GLIBC_TUNABLES not cleared\n", off);
-      return 1;
-    }
-  return 0;
-#endif
 }
 
 static int