Replace std::call_once with llvm:call_once

Summary:
This is required on some platforms, as GNU libstdc++ std::call_once is known to be buggy.

This fixes operation of LLD on at least NetBSD and perhaps OpenBSD and Linux PowerPC.

The same change has been introduced to LLVM and LLDB.

Reviewers: ruiu

Reviewed By: ruiu

Subscribers: emaste, #lld

Tags: #lld

Differential Revision: https://reviews.llvm.org/D33508

llvm-svn: 303788