RISC-V: KVM: Delegate illegal instruction fault to VS mode

Delegate illegal instruction fault to VS mode by default to avoid such
exceptions being trapped to HS and redirected back to VS.

The delegation of illegal instruction fault is particularly important
to guest applications that use vector instructions frequently. In such
cases, an illegal instruction fault will be raised when guest user thread
uses vector instruction the first time and then guest kernel will enable
user thread to execute following vector instructions.

The fw pmu event counter remains undeleted so that guest can still query
illegal instruction events via sbi call. Guest will only see zero count
on illegal instruction faults and know 'firmware' has delegated it.

Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Xu Lu <luxu.kernel@bytedance.com>
Link: https://lore.kernel.org/r/20250714094554.89151-1-luxu.kernel@bytedance.com
Signed-off-by: Anup Patel <anup@brainfault.org>

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