x86-urgent-2020-11-01

 - Handle the BTF bit correctly so it doesn't get lost due to a kernel #DB

 - Only clear and set the virtual DR6 value used by ptrace on user space
   triggered #DB. A kernel #DB must leave it alone to ensure data
   consistency for ptrace.

 - Make the bitmasking of the virtual DR6 storage correct so it does not
   lose DR_STEP.