sys/syz-sysgen: restore detection of unsupported kvm syscalls
sys/syz-sysgen: restore detection of unsupported kvm syscalls

The workaround for broken kvm kernel headers broke detection of unsupported syscalls.
The diagnostic check checks that a syscall is unsupported on all arches,
but arm/riscv kvm syscalls were not noted as unsupported (we dropped them before
giving them to the compiler). As the result it looked like kvm syscalls
are not unsupported on arm/riscv, and we did not produce the diagnostic.

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