compat: make linux/compat.h available everywhere

Parts of linux/compat.h are under an #ifdef, but we end up
using more of those over time, moving things around bit by
bit.

To get it over with once and for all, make all of this file
uncondititonal now so it can be accessed everywhere. There
are only a few types left that are in asm/compat.h but not
yet in the asm-generic version, so add those in the process.

This requires providing a few more types in asm-generic/compat.h
that were not already there. The only tricky one is
compat_sigset_t, which needs a little help on 32-bit architectures
and for x86.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David S. Miller <davem@davemloft.net>