powerpc/book3e: Hack to get gdb moving along on Book3E 64-bit

Our handling of debug interrupts on Book3E 64-bit is not quite
the way it should be just yet. This is a workaround to let gdb
work at least for now. We ensure that when context switching,
we set the appropriate DBCR0 value for the new task. We also
make sure that we turn off MSR[DE] within the kernel, and set
it as part of the bits that get set when going back to userspace.

In the long run, we will probably set the userspace DBCR0 on the
exception exit code path and ensure we have some proper kernel
value to set on the way into the kernel, a bit like ppc32 does,
but that will take more work.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>