For the x86-64 JIT this is implemented with pulling rdtscp's result for
this value.
For Interpreter and AArch64 JIT this is implemented with the getcpu
syscall.
Theoretically AArch64 could implement this with MPIDR_EL1 but because
SoC vendors hecked this up, we can't. Thanks.
Kernel just returns zero + reserved bits if you try reading it.