UPSTREAM: 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.

Bug: 196480993
(cherry picked from commit 202bf8d7)
Reviewed-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Lecopzer Chen <lecopzer.chen@mediatek.com>
Change-Id: Icfc6e3c8edaed66740eefa9a9cc6d80f83b11069