powerpc/kuap: Wire-up KUAP on 40x

This adds KUAP support to 40x. This is done by checking
the content of SPRN_PID at the time user pgtable is loaded.

40x doesn't have KUEP, but KUAP implies KUEP because when the
PID doesn't match the page's PID, the page cannot be read nor
executed.

So KUEP is now automatically selected when KUAP is selected and
disabled when KUAP is disabled.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/aaefa91897ddc42ac11019dc0e1d1a525bd08e90.1634627931.git.christophe.leroy@csgroup.eu