Add popcount intrinsic

[sbastrakov]

This PR implements an alpakafied popcount() to be used in kernels. It falls back to CUDA/HIP built-ins on GPUs, tries to use compiler-specific intrinsic on CPUs, and has a fallback standard library implementation.

The lonely popcount() as of now may seem not worthy to create another directory and concept. I made it assuming the list may be extended in the future, and this sets an example for such extensions. The choice of popcount is since it is required for this work by @bernhardmgruber , and I think generally useful. However, we may think of putting it in some other way.

I added a unit test. Works on my machine with Visual Studio, I assume the tests would catch if something is wrong for other platforms.

Not sure where to put it in the docs.

Note: I choose name popcount as it would be named so in standard C++20.

[sbastrakov]

@psychocoderHPC @BenjaminW3 do you know if there is some issue with macos CI? It shows to have failed, but no details given. (Unlike the actual issues shown in my other PR).

[BenjaminW3]

yes, github seems to have some issues. Sometimes some builds do not start on macos. You can ignore them and merge either way.

[sbastrakov]

Okay, now waiting for travis so far, but good to know, thanks for info.

[sbastrakov]

Now the failing CI for HIP gives

CMake Error at cmake/VerifyCompiler.cmake:24 (find_package):
Could not find a package configuration file provided by "hip" with any of
the following names:
hipConfig.cmake
hip-config.cmake

I don't think this is the problem on this PR's side.

[sbastrakov]

Oh, I meant to re-run one job and it restarted all of them

[psychocoderHPC]

Now the failing CI for HIP gives

CMake Error at cmake/VerifyCompiler.cmake:24 (find_package):
Could not find a package configuration file provided by "hip" with any of
the following names:
hipConfig.cmake
hip-config.cmake

I don't think this is the problem on this PR's side.

I restarted the job

[sbastrakov]

I believe the same HIP travis job failed again with the same message @psychocoderHPC .

[psychocoderHPC]

@sbastrakov I updated our HIP requirements in #1006, I hope this will solve the issue.

[BenjaminW3]

@sbastrakov Please rebase to get the build green again.