Add EXECUTORCH_LOG_LEVEL option to CMake (#2532)

Summary:
Add a CMake setting (EXECUTORCH_LOG_LEVEL) to set ET_MIN_LOG_LEVEL. Must be one of Debug, Info, Error, or Fatal (case-insensitive).

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Pull Request resolved: https://github.com/pytorch/executorch/pull/2532

Test Plan: Ran xnn_executor_runner with EXECUTORCH_LOG_LEVEL set to "debug", "Debug", "info", and "bad". Log level and build messages behaved as expected.

Reviewed By: digantdesai

Differential Revision: D55147165

Pulled By: GregoryComer

fbshipit-source-id: 3bab69948dea4c81cc3e7a46054b62cfb8cc6994