powerpc/sstep: Let compute tests specify a required cpu feature

An a array of struct compute_test's are used to declare tests for
compute instructions. Add a cpu_feature field to struct compute_test as
an optional way to specify a cpu feature that must be present. If not
present then skip the test.

Signed-off-by: Jordan Niethe <jniethe5@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200525025923.19843-4-jniethe5@gmail.com