Fix initialisation order for ID register fields.

Making the constructor `constexpr` gives these "constant" rather than
"dynamic" initialisation, which guarantees that they'll be initialised
before any other dynamically-initialised static variables.

This makes it safe to make static variable initialisers dependent on the
result of `CPUFeatures::InferFromOS()` and similar functions.

Change-Id: Ib65e92273c7ac03d07e17cbe844b598ef3bc60ac