selftests/powerpc: Add test for execute-disabled pkeys

Apart from read and write access, memory protection keys can
also be used for restricting execute permission of pages on
powerpc. This adds a test to verify if the feature works as
expected.

Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200604125610.649668-4-sandipan@linux.ibm.com