Linux: Deprecate <sys/sysctl.h> and sysctl

Now that there are no internal users of __sysctl left, it is possible
to add an unconditional deprecation warning to <sys/sysctl.h>.

To avoid a test failure due this warning in check-install-headers,
skip the test for sys/sysctl.h.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

scripts/check-installed-headers.sh

@@ -53,7 +53,6 @@ trap "rm -f '$cih_test_c'" 0
 
 failed=0
 is_x86_64=unknown
-is_x32=unknown
 for header in "$@"; do
     # Skip various headers for which this test gets a false failure.
     case "$header" in
@@ -75,27 +74,10 @@ for header in "$@"; do
         (finclude/*)
             continue;;
 
-	# sys/sysctl.h is unsupported for x32.
+	# sys/sysctl.h produces a deprecation warning and therefore
+	# fails compilation with -Werror.
 	(sys/sysctl.h)
-            case "$is_x32" in
-                (yes) continue;;
-                (no)  ;;
-                (unknown)
-                    cat >"$cih_test_c" <<EOF
-#if defined __x86_64__ && defined __ILP32__
-# error "is x32"
-#endif
-EOF
-                    if $cc_cmd -fsyntax-only "$cih_test_c" > /dev/null 2>&1
-                    then
-                        is_x32=no
-                    else
-                        is_x32=yes
-                        continue
-                    fi
-                ;;
-            esac
-	    ;;
+	    continue;;
 
         # sys/vm86.h is "unsupported on x86-64" and errors out on that target.
         (sys/vm86.h)