[cudnn] Add /usr/ to find_library for cudnn for ArchLinux

[umar456]

cuDNN is installed in the usr directory in Arch Linux. The find_library function cannot find cuDNN because it doesn't look in that directory

• What does your PR fix?

  Fixes [cuDNN] build failure on x64-linux #24662

• Which triplets are supported/not supported? Have you updated the CI baseline?

  This change only applies to x64-linux systems

• Does your PR follow the maintainer guide?

Yes

• If you have added/updated a port: Have you run ./vcpkg x-add-version --all and committed the result?

Yes

If you are still working on the PR, open it as a Draft: https://github.blog/2019-02-14-introducing-draft-pull-requests/

[ghost]

All CLA requirements met.

[Adela0814]

@cenit Can you help confirm if this change is reasonable?

[cenit]

seems right!

[dg0yt]

I get the impression that the actual root problem is that this port should use a CMakeLists.txt and operate in project mode, instead of implementing everything in portfile.cmake, i.e. script mode.
(Doesn't portfile.cmake partially duplicate FindCUDNN.cmake?)

[BillyONeal]

@dg0yt I'm pretty sure this port predates almost any reasonable built-in CMake behavior. It's possible that now we could rely on built-ins and do something like your suggestion here, but I don't think we need to suggest such a complete overhaul here when this will work.

@umar456 Thanks for the bug fix! I agree with @dg0yt that using more standard ways to get at this stuff would be nice if you'd be willing to contribute such a change as a followup PR.