target-i386: Re-introduce optimal breakpoint removal

Before the last patch, we had an efficient loop that disabled
local breakpoints on task switch.  Re-add that, but in a more
general way that handles changes to the global enable bits too.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
