Swift CI updated to Xcode 12 beta 3 and macOS 10.15.6

[benrimmington]

@mishal_shah Will version 3.18.1 of CMake also be required?

Compiling Swift with --xcode option on Xcode 12 beta 3

When trying to build with Xcode 12b3, it tries to build for arm64 for some reason. I've found this issue in Cmake - Xcode 12 Universal Beta default architecture selection (#20893) · Issues · CMake / CMake · GitLab and added --extra-cmake-options="-DCMAKE_OSX_ARCHITECTURES=x86_64". This worked for Cmark and LLVM, but Swift itself still builds for arm64 and fails when linking with LLVM.