sparc: move generic-y of exported headers to uapi/asm/Kbuild

Since commit fcc8487d

 ("uapi: export all headers under uapi
directories"), all (and only) headers under uapi directories are
exported, but asm-generic wrappers are still exceptions.

To complete de-coupling the uapi from kernel headers, move generic-y
of exported headers to uapi/asm/Kbuild.

With this change, "make headers_install" will just need to parse
uapi/asm/Kbuild to build up exported headers.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>