[compiler-rt] [compiler-rt] [CMake] Add option to disable sanitizer-common builds by default (PR #160962)

[Dan Blackwell via llvm-commits]

https://github.com/DanBlackwell requested changes to this pull request.

I think adding the new toggle is a good idea. Can you please check whether the current boolean logic makes sense (I've added a comment). 

Also, my understanding is that this change won't affect the default behaviour (i.e. the flag is ON by default), can you update the title of the PR to remove the 'by default' if that's the case.

https://github.com/llvm/llvm-project/pull/160962