KVM: arm64: Handle FEAT_IDST for sysregs without specific handlers

KVM: arm64: Handle FEAT_IDST for sysregs without specific handlers

Add a bit of infrastrtcture to triage_sysreg_trap() to handle the
case of registers falling into the Feature ID space that do not
have a local handler.

For these, we can directly apply the FEAT_IDST semantics and inject
an EC=0x18 exception. Otherwise, an UNDEF will do.

Reviewed-by: Joey Gouly <joey.gouly@arm.com>
Reviewed-by: Yuan Yao <yaoyuan@linux.alibaba.com>
Link: https://patch.msgid.link/20260108173233.2911955-5-maz@kernel.org
Signed-off-by: Marc Zyngier <maz@kernel.org>