hyperv: make Hyper-V driver compiled as .ko file

[howard0su_gmail.com]

Summary

1. Move all hyperv related logic into dev/hyperv
2. Expose doreti as a global symbol from exception.S
3. Remove option HYPERV, leave device hyperv

[jhb]

Definitely a fan of this change.

sys/amd64/amd64/exception.S

660 ↗

(On Diff #15078)

This part is definitely fine (and kib@ ok'd it to)

[kib]

sys/amd64/amd64/exception.S

660 ↗

(On Diff #15078)

We usually spell it .globl.

[jhb]

I've committed the part to expose doreti as I think it warrants its own commit. Sepherosa, would you like to commit the rest of this change?

[sepherosa_gmail.com]

In D5910#126628, @jhb wrote:

I've committed the part to expose doreti as I think it warrants its own commit. Sepherosa, would you like to commit the rest of this change?

I talked to Howard about committing this patch yesterday. He told me to hold on for a while.