CPUs don't generally expose an interface that lets you access the microprogramming instructions directly - indeed there's no binary format for those instructions. So there'd be no way a compiler could target them and no point in it doing so as noone would be able to use the resulting binary.
Even if it were possible, it wouldn't be a great idea - the ISA is the standard that is guaranteed to remain backwards compatible, whereas the internal microcode is available for the processor vendor to change whenever it is convenient. It is also likely to be a lot less compressed than the ISA, so binaries would be bigger and you'd lose out on performance in the instruction cache.