[OpenMP][FIX] Avoid a race between initialization and first state reads

When we pick state 0 to initialize state but thread N is going to be the
"main thread", in generic mode, we would require extra synchronization.
Instead, we should pick the main thread to initialize state in generic
mode and any thread in SPMD mode.

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D112874