Correct types of pointers to doacross_num_done

This field is defined as kmp_int32, so we should use neither
pointers to kmp_int64 nor 64 bit atomic instructions.
(Found while testing on a Raspberry Pi, 32 bit ARM)

Differential Revision: https://reviews.llvm.org/D41656

git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@321964 91177308-0d34-0410-b5e6-96231b3b80d8