ppc: Better figure out if processor has HV mode

ppc: Better figure out if processor has HV mode

We use an env. flag which is set to the initial value of MSR_HVB in
the msr_mask. We also adjust the POWER8 mask to set SHV.

Also use this to adjust ctx.hv so that it is *set* when the processor
doesn't have an HV mode (970 with Apple mode for example), thus enabling
hypervisor instructions/SPRs.

Signed-off-by: Benjamin Herrenschmidt <[email protected]>
Reviewed-by: David Gibson <[email protected]>
[clg: ctx.hv used to be defined only for the hypervisor kernel
      (HV=1|PR=0). It is now defined also when PR=1 and conditions are
      fixed accordingly.
      stripped unwanted tabs.]
Signed-off-by: Cédric Le Goater <[email protected]>
Signed-off-by: David Gibson <[email protected]>