x86: set bp field in pt_regs properly

Save rbp twice: One is for marking the stack frame, as usual (already
there), and the other, to fill pt_regs properly. This is because bx
comes right before the last saved register in that structure, and not
bp. If the base pointer were in the place bx is today, this would not
be needed.

Signed-off-by: Glauber Costa <gcosta@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>