uapi: export all arch specifics directories

This patch removes the need of subdir-y. Now all files/directories under
arch/<arch>/include/uapi/ are exported.

The only change for userland is the layout of the command 'make
headers_install_all': directories asm-<arch> are replaced by arch-<arch>/.
Those new directories contains all files/directories of the specified arch.

Note that only cris and tile have more directories than only asm:
 - arch-v[10|32] for cris;
 - arch for tile.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>