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.

Change-Id: I67d31c0f32a9182be25a1a6c56512b51e85da9b2
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>
Git-commit: 54b880caf15034644b564e378abf67b7f9eaf4dc
Git-repo: https://source.codeaurora.org/quic/la/kernel/msm-4.14/


[schikk@codeaurora.org: resolve trivial merge conflicts]
Signed-off-by: Swetha Chikkaboraiah <schikk@codeaurora.org>