hyperv: Implement "enlightened" time counter, which is rdtsc based.

Reviewed by:    kib
Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D8763

hyperv: Allow userland to ro-mmap reference TSC page

This paves way to implement VDSO for the enlightened time counter.

Reviewed by:    kib
Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D8768