Provide deterministic (and somewhat useful) value for RDPID result,
and for %ecx after RDTSCP.

Initialize TSC_AUX MSR with CPUID. It allows for userspace to cheaply
identify CPU it was executed on some time ago, which is sometimes useful.

Note: The values returned might be changed in future.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week