KVM: arm64: pkvm: Report optional ID register traps with a 0x18 syndrome

KVM: arm64: pkvm: Report optional ID register traps with a 0x18 syndrome

With FEAT_IDST, unimplemented system registers in the feature ID space
must be reported using EC=0x18 at the closest handling EL, rather than
with an UNDEF.

Most of these system registers are always implemented thanks to their
dependency on FEAT_AA64, except for a set of (currently) three registers:
GMID_EL1 (depending on MTE2), CCSIDR2_EL1 (depending on FEAT_CCIDX),
and SMIDR_EL1 (depending on SME).

For these three registers, report their trap as EC=0x18 if they
end-up trapping into KVM and that FEAT_IDST is implemented in the guest.
Otherwise, just make them UNDEF.

Link: https://patch.msgid.link/20260108173233.2911955-9-maz@kernel.org
Signed-off-by: Marc Zyngier <maz@kernel.org>