arm64: Use the _REG macros to read ID registers

[andrew]

Summary

To build with old toolchains use the *_REG macros to access the ID
registers. These become a name in the form S?_?_C?_C?_? where the '?'
values encode the op and CR values needed to access the register.

For consistency use these macros for all ID registers, even if most
toolchains understand them.

Sponsored by: Arm Ltd