kbuild: verify asm-generic header list

In order to integrate the system call header generation with generating
the asm-generic wrappers, restrict the generated headers to those that
actually exist in include/asm-generic/.

The path is already known, so add these as a dependency.

The asm-generic/bugs.h header was removed in commit 61235b24 ("init:
Remove check_bugs() leftovers"), which now causes a build failure, so
drop it from the list.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>