x86-64: ptrace generic resume

This removes the handling for PTRACE_CONT et al from the 64-bit
ptrace code, so it uses the new generic code via ptrace_request.

Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>