untangle the <asm/siginfo.h> mess

[Christoph Hellwig]

We currently have uapi versions of <asm/siginfo.h> which at least on a few
architectures define bits of the userspace ABI before including an uapi
asm-generic version, and also non-uapi versions which do almost nothing
except for making sure other headers include the right uapi one before
including an asm-generic that defined two prototypes and a few defines
that aren't architecture specific in any.

This series completely gets rid of the non-uapi versions and cleans up
various small bits on the way.

Note that Thomas has some posix-timers work overlapping with this, so
it might make sense to take these through the tip tree.