kbuild, x86: Track generated headers with generated-y

Track generated header files which aren't already in genhdr-y, alongside
generic-y wrappers in the */include/generated/[uapi/]asm/ directories.
Currently only x86 generates extra headers in these directories, for the
purposes of enumerating system calls for different ABIs, and xen
hypercalls.

This will allow the asm-generic wrapper handling code to remove stale
wrappers when files are removed from generic-y, without also removing
these headers which are generated separately.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-kbuild@vger.kernel.org
Cc: linux-doc@vger.kernel.org
Cc: Michal Marek <mmarek@suse.com>
Link: http://lkml.kernel.org/r/1466808144-23209-2-git-send-email-james.hogan@imgtec.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

Documentation/kbuild/makefiles.txt

@@ -47,6 +47,7 @@ This document describes the Linux kernel Makefiles.
 		--- 7.2 genhdr-y
 		--- 7.3 destination-y
 		--- 7.4 generic-y
+		--- 7.5 generated-y
 
 	=== 8 Kbuild Variables
 	=== 9 Makefile language
@@ -1319,6 +1320,19 @@ See subsequent chapter for the syntax of the Kbuild file.
 		Example: termios.h
 			#include <asm-generic/termios.h>
 
+	--- 7.5 generated-y
+
+	If an architecture generates other header files alongside generic-y
+	wrappers, and not included in genhdr-y, then generated-y specifies
+	them.
+
+	This prevents them being treated as stale asm-generic wrappers and
+	removed.
+
+		Example:
+			#arch/x86/include/asm/Kbuild
+			generated-y += syscalls_32.h
+
 === 8 Kbuild Variables
 
 The top Makefile exports the following variables:

arch/x86/include/asm/Kbuild

@@ -1,5 +1,11 @@
 
 
+generated-y += syscalls_32.h
+generated-y += syscalls_64.h
+generated-y += unistd_32_ia32.h
+generated-y += unistd_64_x32.h
+generated-y += xen-hypercalls.h
+
 genhdr-y += unistd_32.h
 genhdr-y += unistd_64.h
 genhdr-y += unistd_x32.h