x86: cpufunc: Add rdtscp_aux()

Add a variant of 'rdtscp()' that retains and returns the 'IA32_TSC_AUX'
value read by 'rdtscp'.

Sponsored By: Juniper Networks, Inc.
Sponsored By: Klara, Inc.
Reviewed by: markj, kib
Differential Revision: https://reviews.freebsd.org/D31415