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
diff --git a/lldb/source/Plugins/Process/FreeBSD/ProcessFreeBSD.cpp b/lldb/source/Plugins/Process/FreeBSD/ProcessFreeBSD.cpp
index b26e3ee..1a3ac23 100644
--- a/lldb/source/Plugins/Process/FreeBSD/ProcessFreeBSD.cpp
+++ b/lldb/source/Plugins/Process/FreeBSD/ProcessFreeBSD.cpp
@@ -46,6 +46,8 @@
 
 #include "lldb/Host/posix/Fcntl.h"
 
+#include "llvm/Support/Threading.h"
+
 using namespace lldb;
 using namespace lldb_private;
 
@@ -71,9 +73,9 @@
 }
 
 void ProcessFreeBSD::Initialize() {
-  static std::once_flag g_once_flag;
+  static llvm::once_flag g_once_flag;
 
-  std::call_once(g_once_flag, []() {
+  llvm::call_once(g_once_flag, []() {
     PluginManager::RegisterPlugin(GetPluginNameStatic(),
                                   GetPluginDescriptionStatic(), CreateInstance);
     ProcessPOSIXLog::Initialize(GetPluginNameStatic());