Add a unit test stub for system_thread_set_state

[metcalf]

Problem

The existing unit test stubs do not include a definition for system_thread_set_state. I'm working on unit testing a Particle project and the lack of this stub causes the following error if I call SYSTEM_THREAD(ENABLED); in the code under test:

dyld: lazy symbol binding failed: Symbol not found: _system_thread_set_state
  Referenced from: /private/var/tmp/_bazel_andrew/a4c7ff0b81f89f303b386f9cf19fd00e/sandbox/darwin-sandbox/570/execroot/__main__/bazel-out/darwin_arm64-fastbuild/bin/tests/tests.runfiles/__main__/tests/../_solib_darwin_arm64//libsrc_Slibparticle-bazel-demo.so
  Expected in: flat namespace

dyld: Symbol not found: _system_thread_set_state
  Referenced from: /private/var/tmp/_bazel_andrew/a4c7ff0b81f89f303b386f9cf19fd00e/sandbox/darwin-sandbox/570/execroot/__main__/bazel-out/darwin_arm64-fastbuild/bin/tests/tests.runfiles/__main__/tests/../_solib_darwin_arm64//libsrc_Slibparticle-bazel-demo.so
  Expected in: flat namespace

Solution

Add a no-op stub for system_thread_set_state

Steps to Test

I wrote a demo project to use Bazel and Google Test with Particle for unit testing. To make it compile successfully, I had to include the changes from this PR as a patch:
https://github.com/metcalf/arduino-particle-bazel-demo/blob/main/tools/io_particle_device_os.patch

Example App

N/A

References

Links to the Community, Docs, Other Issues, etc..

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Completeness

• User is totes amazing for contributing!
• Contributor has signed CLA (Info here)
• Problem and Solution clearly stated
• Run unit/integration/application tests on device
• Added documentation
• Added to CHANGELOG.md after merging (add links to docs and issues)