compat: provide compat_ptr() on all architectures

In order to avoid needless #ifdef CONFIG_COMPAT checks,
move the compat_ptr() definition to linux/compat.h
where it can be seen by any file regardless of the
architecture.

Only s390 needs a special definition, this can use the
self-#define trick we have elsewhere.

Reviewed-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>