[wip] Fix CMAKE_SYSTEM_PROCESSOR (Resubmission of #4688)

[vicroms]

Related PRs:
#4970
#4688
#5218

#4688 was reverted due to regressions

Quoting @UnaNancyOwen:

In case of UWP and Linux triplets, CMAKE_SYSTEM_PROCESSOR is not specified. It is empty.
For details this problems, Please refer to this comment #4635 (comment).
This pull request will set CMAKE_SYSTEM_PROCESSOR under UWP and Linux triplets.
If you want to explicitly specified CPU arcitecture, You can use VCPKG_CMAKE_SYSTEM_PROCESSOR in triplet files.

[vicroms]

Current Regressions Status

Port	Regressions
anax	x86-windows
caffe2	x64-linux
darknet	x64-windows + x64-windows-static + x86-windows
ffmpeg	x64-windows + x64-uwp
icu	x64-windows
libvpx	x64-windows + x64-uwp
glew	x64-windows-static
netcdf-c	x64-windows-static
octomap	x64-windows + x86-windows
openssl-uwp	x64-uwp + arm-uwp
openssl-windows	x64-windows + x64-windows-static + x86-windows
tbb	x64-windows + x64-windows-static + x86-windows
x264	x64-windows + x64-uwp
vxl	x64-linux
z3	x64-linux

ffmpeg, icu, libvpx, and x264: failed acquiring msys pacman packages.
openssl-windows, tbb: Fails vcpkg_test_cmake(), test CMake function has bug on 64-bits, need to check logs for exact error message. Should be fixed in master
vxl: Requires built-in OpenJpeg which was already installed in full rebuild. (Rerun with --exclude openjpeg)

[cenit]

For darknet static I can open a PR. It is just a matter of a bug in the findpthreads_windows.cmake which is fixed in a PR upstream ( we are still testing it) and which I can submit also here. It should not have passed CI tests for static from the beginning...

[cenit]

also many of those ports are fixed in my #5169 PR, if you don't want to duplicate efforts

[vicroms]

@cenit
Ok! Let's work on getting #5169 merged!