MFC r343859:
Add comment noting that the strange spelling of GenuineIntel is for reason.

MFC r322456:
On i386 with CPUID but without SSE2, set lfence_works to LMB_NONE
instead of looping.

MFC r321608:
Use MFENCE to serialize RDTSC on non-Intel CPUs.

MFC r321652:
Simplify flow control.

MFC r314319 (by oshogbo):
Don't try to open devices in the gettc() function which will always
fail in the Capability mode. Instead silently fallback to the syscall
method, which is done for example in the gettimeofday(2) function.

MFC r314320 (by oshogbo):
Remove unneeded variable initialization from r314319.

MFC r321461:
Fix indent.

MFC r312418,r312422:

r312418:

Conditionalize hyperv support in gettimeofday(2) based on MK_HYPERV

The effect at runtime is negligible as the hyperv timer isn't available
except when hyperv is loaded.

This is a prerequisite for conditionalizing the header build/install out
of the build

r312422:

Only conditionally add in hyperv support if we're building amd64

This unbreaks the build because the assembly is written for x64.

Pointyhat to: ngie

MFC r311287:
__vdso_gettc(): be extra careful with /dev/hpet mappings, never unmap
the mapping which might be accessed by other threads.

MFC 310048,310101,310239

310048
hyperv: Implement "enlightened" time counter, which is rdtsc based.

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

310101
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

310239
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