[lldb] Replace std::call_once() with llvm::call_once()

Remove the single instance of std::call_once() in lldbTarget library
with llvm::call_once().  The former fails to build on NetBSD when
combined with llvm::once_flag (which replaced std::once_flag
in r369618), and combining the two is probably generally incorrect
anyway.

llvm-svn: 370748