selftests/powerpc: Add a test of 4PB SLB handling

Add a test for a bug we had in the 4PB address space SLB handling. It
was fixed in commit 4c2de74c ("powerpc/64: Interrupts save PPR on
stack rather than thread_struct").

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220317143925.1030447-1-mpe@ellerman.id.au