cmake: add USE_SYSTEM_* options; support find_package(NNPACK)

[ConnorBaker]

Similar to the work done in https://github.com/pytorch/pytorch/pull/37137, this adds the following CMake options:

• USE_SYSTEM_LIBS
• USE_SYSTEM_CPUINFO
• USE_SYSTEM_FP16
• USE_SYSTEM_FXDIV
• USE_SYSTEM_PSIMD
• USE_SYSTEM_PTHREADPOOL
• USE_SYSTEM_GOOGLE_TEST
• USE_SYSTEM_PYTHON

This is particularly useful in the context of Nix, where we can build these libraries once and then re-use them elsewhere to avoid rebuilding vendors dependencies.

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

[ConnorBaker]

Closed in favor of contributing to XNNPACK.