x86/tsc: fetch frequency from CPUID when running on Xen

Introduce a helper to fetch the TSC frequency from CPUID when running
under Xen.

Since the TSC can also be initialized early when running as a Xen
guest pull out the call to tsc_init() from the
early_clock_source_init() handlers and place it in clock_init(), as
otherwise all handlers would call tsc_init() anyway.

Reviewed by: markj
Sponsored by: Citrix Systems R&D
Differential revision: https://reviews.freebsd.org/D34581