Fixed Xcode 12 supported architectures deprecation

[mjjimenez]

Description

• XCode 12 deprecated VALID_ARCHS build setting in Xcode 12. It also included arm64 ios simulator targets for ARM Macs that don't exist yet. EXCLUDED_ARCHS build setting is added to remove that build target for now.
• LibTorch 1.6.1 fixes previous bug with 1.6 where it couldn't be built with a real device as a target

Checklist

• [x] I have followed the Contribution Guidelines and Code of Conduct
• [x] I have commented my code following the OpenMined Styleguide
• [x] I have labeled this PR with the relevant Type labels

[codecov[bot]]

Codecov Report

Merging #185 into dev will decrease coverage by 7.08%. The diff coverage is 15.56%.

@@            Coverage Diff             @@
##              dev     #185      +/-   ##
==========================================
- Coverage   52.97%   45.89%   -7.09%     
==========================================
  Files          16       21       +5     
  Lines        1882     2351     +469     
==========================================
+ Hits          997     1079      +82     
- Misses        885     1272     +387     

Impacted Files	Coverage Δ
SwiftSyft/Classes/SyftPlan.swift	6.66% <0.00%> (-0.89%)	:arrow_down:
SwiftSyft/Classes/TorchWrapper/src/TorchIValue.mm	0.00% <0.00%> (ø)
SwiftSyft/Classes/TorchWrapper/src/TorchModule.mm	0.00% <0.00%> (ø)
SwiftSyft/Classes/TorchWrapper/src/TorchTensor.mm	0.00% <0.00%> (ø)
SwiftSyft/Classes/DataLoader.swift	15.38% <15.38%> (ø)
SwiftSyft/Classes/SwiftSyftError.swift	23.80% <23.80%> (ø)
SwiftSyft/Classes/SignallingMessages.swift	35.03% <50.00%> (ø)
SwiftSyft/Classes/SyftClient.swift	82.13% <67.04%> (+3.41%)	:arrow_up:
SwiftSyft/Classes/NetworkManager.swift	75.00% <100.00%> (+1.92%)	:arrow_up:
SwiftSyft/Classes/SimplePing.m	80.74% <0.00%> (-1.73%)	:arrow_down:
... and 13 more

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