asm-generic: add a generic uaccess.h

Based on discussions with Michal Simek and code
from m68knommu and h8300, this version of uaccess.h
should be usable by most architectures, by overriding
some parts of it.

Simple NOMMU architectures can use it out of
the box, but a minimal __access_ok() should be
added there as well.

Cc: Michal Simek <monstr@monstr.eu>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>