Bits 20 ~ 23 of CPSR are reserved (RAZ, read as zero), but they are not
zero if the arm program runs on aarch64-linux. AArch64 tracer gets PSTATE
from arm 32-bit tracee as CPSR, but bits 20 ~ 23 are used in PSTATE. I
think kernel should clear these bits when it is read through ptrace, but
the fix in user space is still needed.