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

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

BugLink: https://bugs.launchpad.net/bugs/1959376
[ Upstream commit a8968521cfdc3e339fe69473d6632e0aa8d7202a ]

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
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>