kbuild: introduce HDR_ARCH_LIST for headers_install_all

Using HDR_ARCH_LIST you can specify subset of architectures you want to get
headers for.

Signed-off-by: Kirill A. Shutemov <kirill@shutemov.name>
Signed-off-by: Michal Marek <mmarek@suse.cz>

diff --cc Documentation/make/headers_install.txt
Simple merge

diff --cc scripts/headers.sh
Simple merge