Currently default simd alignment is defined by Clang specific TargetInfo class. This class cannot be reused for LLVM Flang. That's why default simd alignment calculation has been moved to OMPIRBuilder which is common for Flang and Clang.
Previous attempt: https://reviews.llvm.org/D138496 was wrong because the default alignment depended on the number of built LLVM targets.
If we wanted to calculate the default alignment for PPC and we hadn't specified PPC LLVM target to build, then we would get 0 as the alignment because OMPIRBuilder couldn't create PPCTargetMachine object and it returned 0 as the default value. If PPC LLVM target had been built earlier, then OMPIRBuilder could have created PPCTargetMachine object and it would have returned 128.