[OpenMP] libomp: fix build by cl with vs2019

Replace VLA with dynamic allocation using alloca().
This fixes https://bugs.llvm.org/show_bug.cgi?id=48919.

Differential Revision: https://reviews.llvm.org/D95627

GitOrigin-RevId: 7f5ad0e07162e0c19e569986ee37a17c147c9a27
1 file changed