kbuild: drop include/asm

We no longer use this directory for generated files and
all architectures has moved their header files so no
symlink tricks are needed either.

Drop the symlink and drop the ARCH check.

If we really need to check that the SRCARCH has not changed
when we build a kernel we can add this check back - but then we will
find a more convenient way to store the info.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>