linux: Add process_madvise

It was added on Linux 5.10 (ecb8ac8b1f146915aa6b96449b66dd48984caacc)
with the same functionality as madvise but using a pidfd of the target
process.

Checked on x86_64-linux-gnu and i686-linux-gnu.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

sysdeps/unix/sysv/linux/bits/mman_ext.h

@@ -0,0 +1,33 @@
+/* System-specific extensions of <sys/mman.h>, Linux version.
+   Copyright (C) 2022 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/>.  */
+
+#ifndef _SYS_MMAN_H
+# error "Never include <bits/mman_ext.h> directly; use <sys/mman.h> instead."
+#endif
+
+#ifdef __USE_GNU
+#include <bits/types/struct_iovec.h>
+
+/*
+ */
+extern __ssize_t process_madvise (int __pid_fd, const struct iovec* __iov,
+				  size_t __count, int __advice,
+				  unsigned __flags)
+  __THROW;
+
+#endif /* __USE_GNU  */