OpenMP RTL cleanup: eliminated warnings with -Wcast-qual, patch 2.

Changes are: got all atomics to accept volatile pointers that allowed
to simplify many type conversions. Windows specific code fixed correspondingly.

Differential Revision: https://reviews.llvm.org/D35417


git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@308164 91177308-0d34-0410-b5e6-96231b3b80d8