x86: make TASK_SIZE_MAX usable from assembly code

For 64-bit the only thing missing was a strategic _AC, and for 32-bit we
need to use __PAGE_OFFSET instead of PAGE_OFFSET in the TASK_SIZE
definition to escape the explicit unsigned long cast.  This just works
because __PAGE_OFFSET is defined using _AC itself and thus never needs
the cast anyway.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>