illumos gate

This change has been in SmartOS and OmniOS for over a year now. When initially merged into SmartOS, Hans mentioned his testing efforts:

I tested this by controlling a VM running Ubuntu 17.10. I have verified that I can single step from the first instruction of the UEFI ROM in real mode through the initial setup for protected mode and later into long mode. I have verified that in each situation the register dumps look the way they are supposed to, that the right disassembler is chosen based on the CS attributes, and that disassembling and stack tracing uses the right segments when appropriate. I've also verified that stopping/resuming VM execution and accessing memory and register contents still work as expected once the OS was booted, and that the OS wasn't doing anything unexpected while single-stepping through it's idle code.

With the upstreaming patch applied, I booted up a guest instance and checked similar functionality:
- The guest remained paused when started with the -d flag
- I was able to single step through instructions, starting with the bootloader
- When released with :c, the guest booted up fine
- I could observe system state via dcmds like ::sysregs

And additional note on testing: After merging this into SmartOS, no regressions were noted in unrelated mdb operations. The minor restructuring in the patch does not appear to have adversely effected other unrelated operations.