kbuild: move asm-offsets.h to include/generated

The simplest method was to add an extra asm-offsets.h
file in arch/$ARCH/include/asm that references the generated file.

We can now migrate the architectures one-by-one to reference
the generated file direct - and when done we can delete the
temporary arch/$ARCH/include/asm/asm-offsets.h file.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Michal Marek <mmarek@suse.cz>