Fix the MinGW builder.  Apparently std::call_once and
std::recursive_mutex are not available on MinGW and breaks the
builder.  Revert to using a function local static and sys::Mutex
just to get the tree green until we figure out a better solution.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211424 91177308-0d34-0410-b5e6-96231b3b80d8