Add identcpu.c and specialreg.h definitions for the new Intel/AMD MSRs

Add identcpu.c and specialreg.h definitions for the new Intel/AMD MSRs
that should help mitigate spectre. This is just the detection piece, these
features are not yet used.

Part of a larger ongoing effort to mitigate meltdown/spectre. i386 will
come later; it needs some machdep.c cleanup first.

ok kettenis@