cmake: support find_package(XNNPACK)

[ConnorBaker]

Adds a CMake configuration file to make it possible for CMake-based projects to use find_package to consume this library.

Part of the work being done for NixOS/nixpkgs#239291.

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[ConnorBaker]

@Maratyszcza @alankelly would either of you have a chance to review this?

[ConnorBaker]

Rebased, squashed, and force-pushed!

@Maratyszcza I think I answered your comments, please let me know if you have any more questions.

I apologize for the delay in getting back to you!

[waterdropw]

I very much agree with this PR, because if i just use "xnnpack.h" and "libXNNPACK.a" after installed into /usr/local , then a lot of "undefined reference... " build error will occur!

[ConnorBaker]

Quoted the path names to avoid weirdness when interpolating strings. (Yay, strings!)

Sorry for the horrible average turn around time!