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

hyperv: Implement userspace gettimeofday(2) with Hyper-V reference TSC

This 6 times gettimeofday performance, as measured by
tools/tools/syscall_timing

Reviewed by:    kib
Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D8789