mmix: Don't turn off .hidden support.

In 2012 e2769e908a915ebbc/r192344, I added the following lines, that I
now delete.  I've changed my mind: ELF-related targets based on gas,
that support only static linking, have .hidden "for free", regardless
of the visibility of the hidden object in the linked executable.  No
regressions for mmix-knuth-mmixware and "fixes":

FAIL: gcc.dg/torture/pr94947-1.c   -O2 -flto -fuse-linker-plugin -fno-fat-lto-objects  (test for excess errors)
Excess errors:
lto1: warning: visibility attribute not supported in this configuration; ignored [-Wattributes]

...a warning which is emitted for "all" LTO tests (of which most have
warnings filtered out); LTO "thinks" all supported targets have
.hidden support.

gcc:
	* configure.ac (out-of-tree linker .hidden support): Don't turn off
	for mmix-knuth-mmixware.
	* configure: Regenerate.

gcc/configure

@@ -23471,12 +23471,6 @@ else
   if test x"$ld_is_gold" = xyes; then
     :
   elif echo "$ld_ver" | grep GNU > /dev/null; then
-    case "${target}" in
-      mmix-knuth-mmixware)
-        # The linker emits by default mmo, not ELF, so "no" is appropriate.
-	gcc_cv_ld_hidden=no
-	;;
-    esac
     if test 0"$ld_date" -lt 20020404; then
       if test -n "$ld_date"; then
 	# If there was date string, but was earlier than 2002-04-04, fail

gcc/configure.ac

@@ -2874,12 +2874,6 @@ else
   if test x"$ld_is_gold" = xyes; then
     :
   elif echo "$ld_ver" | grep GNU > /dev/null; then
-    case "${target}" in
-      mmix-knuth-mmixware)
-        # The linker emits by default mmo, not ELF, so "no" is appropriate.
-	gcc_cv_ld_hidden=no
-	;;
-    esac
     if test 0"$ld_date" -lt 20020404; then
       if test -n "$ld_date"; then
 	# If there was date string, but was earlier than 2002-04-04, fail