kbuild: Add support for an "archheaders" target

Add support for an "archheaders" target.  This target can generate
files that need to be installed for user space by "make
headers_install" or "make headers_install_all".

In order to support "make headers_install_all", it must be able to run
without the tree having to be configured first.

Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Michal Marek <mmarek@suse.cz>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

scripts/headers.sh

@@ -7,6 +7,7 @@ set -e
 do_command()
 {
 	if [ -f ${srctree}/arch/$2/include/asm/Kbuild ]; then
+		make ARCH=$2 KBUILD_HEADERS=$1 archheaders
 		make ARCH=$2 KBUILD_HEADERS=$1 headers_$1
 	else
 		printf "Ignoring arch: %s\n" ${arch}