*Ramaphosa urges global efforts to debunk white persecution myths * Trump boycotted G20 summit over alleged mistreatment of whites * Ramaphosa: supremacy ideas are harming national security JOHANNESBURG, Dec 8 (Reuters) - Ideas of white racial superiority are a threat to South Africa's post-apartheid unity, its sovereignty and its diplomatic relations, President Cyril Ramaphosa said on Monday, calling for global efforts to debunk false stories about white persecution. Ramaphosa was speaking at a conference of the co-ruling African National Congress (ANC), the party of Nelson Mandela that defeated racist white minority rule in South Africa three decades ago. "Some in...