powerpc: Use correct procedure call standard for getrandom vDSO call (bug 32440)

A plain indirect function call does not work on POWER because
success and failure are signaled through a flag register, and
not via the usual Linux negative return value convention.

This has potential security impact, in two ways: the return value
could be out of bounds (EAGAIN is 11 on powerpc6le), and no
random bytes have been written despite the non-error return value.

Fixes commit 461cab1de7 ("linux: Add
support for getrandom vDSO").

Reported-by: Ján Stanček <jstancek@redhat.com>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>

stdlib/tst-getrandom-errno.c

@@ -0,0 +1,37 @@
+/* Test errno handling in getrandom (bug 32440).
+   Copyright (C) 2024 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <errno.h>
+#include <stdlib.h>
+#include <support/check.h>
+#include <sys/random.h>
+
+static
+int do_test (void)
+{
+  errno = -1181968554;          /* Just a random value.  */
+  char buf[4];
+  int ret = getrandom (buf, sizeof (buf), -1); /* All flags set.  */
+  if (errno != ENOSYS)
+    TEST_COMPARE (errno, EINVAL);
+  TEST_COMPARE (ret, -1);
+
+  return 0;
+}
+
+#include <support/test-driver.c>