fix: Added a x64 guard to the IL2CPP methods

[bitsandfoxes]

There is something off about the way we call native code on x86 so I'm guarding the IL2CPP feature with UNITY_64 for now:

error LNK2019: unresolved external symbol _il2cpp_gchandle_get_target@4 referenced in function _SentryUnityInfo_il2cpp_gchandle_get_target_m42BE043B96499DAF199C1045023BE6DE1B0FF6EA
  Hint on symbols that are defined and could potentially match:
    _il2cpp_gchandle_get_target

This can easily be reproduced with our own samples and throw_cpp.

[bruno-garcia]

Can we add a 32 bit Unity build to our CI?

[github-actions]

	Fails
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- Added a x64 guard to the IL2CPP methods ([#871](https://github.com/getsentry/sentry-unity/pull/871))

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