Revert 220932.

Commit 220932 caused crash when building clang-tblgen on aarch64 debian target,
so it's blocking all daily tests.

The std::call_once implementation in pthread has bug for aarch64 debian.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221331 91177308-0d34-0410-b5e6-96231b3b80d8