Add __uintr_frame structure and use UIRET instruction for functions with
x86 interrupt calling convention when UINTR is present.
A user interrupt is different than a regular interrupt right? It doesn't make sense that we would change the behavior of the interrupt calling convention just because the the user interrupt instructions are enabled. That would occur just from passing a -march for a newer CPU wouldn't it?
Since there won't be both user interrupt handler and kernel interrupt handler in the source, there is no need for another
attribute. We discussed that kernel might need to use UINTR instructions. We decided that kernel could use inline asm
statements if needed.
So if write kernel code and compile with -march=haswell today, I get IRET. If tomorrow I change my command line to -march=sapphirerapids, now my kernel interrupt code generates user interrupt instructions. That seems surprising.