get rid of the last symlink in uml build

We need to make asm-offsets.h contents visible for objects built
with userland headers.  Instead of creating a symlink, just have the
file with equivalent include (relative to location of header) created
once.  That kills the last symlink used in arch/um builds.

Additionally, both generated headers can become dependencies of
archprepare now, killing the misuse of prepare.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>