execute1: Fix no-op behaviour of reading undefined SPRs

In privileged mode, mfspr from an undefined or unimplemented SPR
number should be a no-op, which is implemented here by writing back
the same value that the destination register previously had.  However,
we ended up writing back 0 because ex1.res2_sel was not set correctly.
To fix this, set res2_sel to 10 in the undefined SPR case.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>

execute1.vhdl

@@ -1456,6 +1456,7 @@ begin
                     end if;
                     slow_op := '1';
                     v.se.noop_spr_read := '1';
+                    v.res2_sel := "10";
                     if ex1.msr(MSR_PR) = '1' then
                         illegal := '1';
                     end if;