[pclmulqdq]

In many modern CPUs, it's an auxiliary processor that "starts at 0" (within its dedicated ROM) and then eventually turns on the main CPUs. In a modern Intel core, I think that CPU is actually the one in the secure enclave, which also happens to do things like DRAM training...

In a microcontroller, the clock generators and the peripherals are often set up by the main core just after boot, and are under user control - the chip's reset network (literally just a wire) handles bringing things into a known state before boot.