target-arm: aarch64: add preliminary support for MSR/MRS

this is very very limited: we only support
setting and getting NZCV (C4.3.10),
getting CTR_EL0 (D8.2.21) with a hardcoded value,
getting DCZID_EL0 (D8.2.23) with a hardcoded value,
getting and setting TPIDR_EL0 (D8.2.87).

Eventually this could be rewritten on top of arm_cp_regs API
with some refactoring necessary (main issue: ARM_CP_64BIT)