Switch std::call_once to llvm::call_once

Summary:
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

Subscribers: #lldb

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D29288

llvm-svn: 294202