kbuild: check asm symlink when building a kernel

We often hit the situation where the asm symlink
in include/ points to the wrong architecture.
In 9 out of 10 cases thats because we forgot to set
ARCH but sometimes we just reused the same tree
for another ARCH. For the merged x86 tree we need
to create a new symlink but this is not obvious.
So with the following patch we check if the symlink
points to the correct architecture and error
out if this is not the case.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>