selftests/powerpc: Add a test of sigreturning to the kernel

We have a general signal fuzzer, sigfuz, which can modify the MSR & NIP
before sigreturn. But the chance of it hitting a kernel address and also
clearing MSR_PR is fairly slim.

So add a specific test of sigreturn to a kernel address, both with and
without attempting to clear MSR_PR (which the kernel must block).

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211209115944.4062384-1-mpe@ellerman.id.au