arm64: enablement for ARM64 in Hyper-V (Part 1)

The changes are to refactor the code of vmbus.c and hyperv.c to keep minimal
arch specific codes there and have them in separate files in x86/ arm64/ .
x86 is a new directory, which contains codes for x86 / x86_64. Instead of
repeating the same codes in existing amd64/ and i386/, this approach reduced
the repetition. This is first of three patches for Hyper-V enablement.

Reviewed by: whu
Tested by: Souradeep Chakrabarti <schakrabarti@microsoft.com>
Sponsored by: Microsoft
Differential Revision: https://reviews.freebsd.org/D36466