Fix ARM64-based CPU detection logic; take #3.

[rolfbjarne]

The previous implementation was broken, because:

• We called GetIsARM64CallingConvention in Runtime's static constructor.
• Runtime.Arch hadn't been set yet in the static constructor, which means it
  was Arch.DEVICE (0).
• This meant GetIsARM64CallingConvention would return true on iOS/tvOS x86-64
  simulator.

So delay the initialization of the IsARM64CallingConvention field until we've
been initialized.

[monojenkins]

Build failure
✅ Build succeeded
✅ API Diff (from stable)
🔥 Failed to compare API and create generator diff 🔥
    Failed to update apidiff references
    Search for Comparing API & creating generator diff in the log to view the complete log.
✅ Test run succeeded

[spouliot]

Did that fail an existing* unit test ? or did you found it in another way/failure ?

• If not then maybe one should be added

[rolfbjarne]

Did that fail an existing* unit test ?

Kind of: monotouch-test started crashing when I put the previous fix in the branch with my other arm64_32 changes (PR #5465).

[rolfbjarne]

API diff failure is due to https://github.com/xamarin/maccore/issues/1452