[kernel][x86] Introduce read_msr_on_cpu

read_msr_on_cpu reads an MSR on a single target CPU. The MSR needs to be
readable without causing a #GP; the read happens in IPI function handler.

ZX-3359

Tested: See 4th patch in patch stack; k cpu rdmsr/wrmsr are added to
exercise these.

Change-Id: I42e6162ba8c2f19872b4d058532b454367e19879