Switch std::call_once to llvm::call_once
The std::call_once implementation in libstdc++ has problems on few systems: NetBSD, OpenBSD and Linux PPC. LLVM ships with a homegrown implementation llvm::call_once to help on these platforms.
This change is required in the NetBSD LLDB port. std::call_once with libstdc++ results with crashing the debugger.
Sponsored by <The NetBSD Foundation>
Reviewers: labath, joerg, emaste, mehdi_amini, clayborg
Reviewed By: labath, clayborg
Differential Revision: https://reviews.llvm.org/D29288