kernel: use IRET instead of SYSRET for switching into usermode

I want to implement a new syscall for returning from "interrupts" (my
crappy take on Plan 9's notes / UNIX signals), and I've realized that
I can't use SYSRET for that, as it clobbers RCX and R11.

This hasn't been an issue so far, as I've only been switching into usermode
to return from syscalls, but now I might be switching into it at arbitrary
moments (right after an interrupt handler recovers from a page fault, etc).

I could make it so I switch between IRET and SYSRET depending on the context,
but I don't think it's worth it.

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