i386/cpu: Save/restore SSP0 MSR for FRED

i386/cpu: Save/restore SSP0 MSR for FRED

Both FRED and CET shadow stack define the MSR MSR_IA32_PL0_SSP (aka
MSR_IA32_FRED_SSP0 in FRED spec).

MSR_IA32_PL0_SSP is a FRED SSP MSR, so that if a processor doesn't
support CET shadow stack, FRED transitions won't use MSR_IA32_PL0_SSP,
but this MSR would still be accessible using MSR-access instructions
(e.g., RDMSR, WRMSR).

Therefore, save/restore SSP0 MSR for FRED.

Tested-by: Farrah Chen <farrah.chen@intel.com>
Signed-off-by: Xin Li (Intel) <xin@zytor.com>
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20251211060801.3600039-14-zhao1.liu@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>