most importantly this gets us
rust-lang/cc-rs#1496 which fixes
oxidecomputer/helios#192 -style miscompilation
of propolis-server (and most other binaries here).

KVM masks DecodeAssists even if nested virtualization is supported (no
mention of DecodeAssists in `svm_set_cpu_caps()`, and all of leaf
8000_000A EDX is cleared by default) so an illumos guest on a Linux host
may see SVM support without DecodeAssist. at first this produced
https://www.illumos.org/issues/16726 but as mentioned there some real
hardware supports SVM without DecodeAssists as well. until
https://www.illumos.org/issues/17543 handles the circumstance in-kernel,
port I/O instructions kick out to userland for emulation.

this implements enough instruction emulation to get illumos guests
booting on non-DecodeAssist illumos hosts, which may themselves be a VM
on KVM or perhaps on an Opteron system.