selftests/powerpc: Skip 4PB test on 4K PAGE_SIZE systems

Systems using the hash MMU with a 4K page size don't support 4PB address
space, so skip the test because the bug it tests for can't be triggered.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220901020215.254097-1-mpe@ellerman.id.au

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 4c2de74cc869 ("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