[build] explicitly set OSX_ARCHITECTURES for targets in stdlib
[build] explicitly set OSX_ARCHITECTURES for targets in stdlib

Our custom build code already sets the architecture of compilation
commands by different means, and so far we were relying on setting
`CMAKE_OSX_ARCHITECTURES` to an empty value to avoid CMake doing the
same.

However, on Apple Silicon, CMake 3.19+ enforces a default value in this
scenario
(https://gitlab.kitware.com/cmake/cmake/-/merge_requests/5291/diffs),
and this would result in the inability to compile code for x86_64.

Addresses SR-14035, rdar://80613594

benlangmuir/swift,parkera/swift,roambotics/swift,parkera/swift,atrick/swift,ahoppen/swift,xwu/swift,roambotics/swift,parkera/swift,atrick/swift,xwu/swift,benlangmuir/swift,ahoppen/swift,apple/swift,hooman/swift,xwu/swift,gregomni/swift,xwu/swift,gregomni/swift,glessard/swift,apple/swift,gregomni/swift,benlangmuir/swift,tkremenek/swift,JGiola/swift,gregomni/swift,gregomni/swift,tkremenek/swift,hooman/swift,roambotics/swift,hooman/swift,xwu/swift,ahoppen/swift,tkremenek/swift,rudkx/swift,parkera/swift,glessard/swift,xwu/swift,parkera/swift,benlangmuir/swift,parkera/swift,apple/swift,roambotics/swift,apple/swift,ahoppen/swift,JGiola/swift,roambotics/swift,xwu/swift,glessard/swift,benlangmuir/swift,parkera/swift,glessard/swift,rudkx/swift,tkremenek/swift,rudkx/swift,hooman/swift,glessard/swift,ahoppen/swift,JGiola/swift,hooman/swift,atrick/swift,atrick/swift,JGiola/swift,rudkx/swift,tkremenek/swift,gregomni/swift,roambotics/swift,tkremenek/swift,hooman/swift,tkremenek/swift,hooman/swift,apple/swift,ahoppen/swift,benlangmuir/swift,rudkx/swift,JGiola/swift,atrick/swift,glessard/swift,atrick/swift,rudkx/swift,parkera/swift,apple/swift,JGiola/swift