kernel/utsname_sysctl.c: print kernel arch

Print the machine hardware name (UTS_MACHINE) in /proc/sys/kernel/arch.

This helps people who debug kernel with initramfs with minimal environment
(i.e.  without coreutils or even busybox) or allow to open sysfs file
instead of run 'uname -m' in high level languages.

Link: https://lkml.kernel.org/r/20220901194403.3819-1-pvorel@suse.cz
Signed-off-by: Petr Vorel <pvorel@suse.cz>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: David Sterba <dsterba@suse.com>
Cc: "Eric W . Biederman" <ebiederm@xmission.com>
Cc: Rafael J. Wysocki <rafael@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/admin-guide/sysctl/kernel.rst

@@ -65,6 +65,11 @@ combining the following values:
 4 s3_beep
 = =======
 
+arch
+====
+
+The machine hardware name, the same output as ``uname -m``
+(e.g. ``x86_64`` or ``aarch64``).
 
 auto_msgmni
 ===========

kernel/utsname_sysctl.c

@@ -73,6 +73,13 @@ static DEFINE_CTL_TABLE_POLL(hostname_poll);
 static DEFINE_CTL_TABLE_POLL(domainname_poll);
 
 static struct ctl_table uts_kern_table[] = {
+	{
+		.procname	= "arch",
+		.data		= init_uts_ns.name.machine,
+		.maxlen		= sizeof(init_uts_ns.name.machine),
+		.mode		= 0444,
+		.proc_handler	= proc_do_uts_string,
+	},
 	{
 		.procname	= "ostype",
 		.data		= init_uts_ns.name.sysname,