Add support for EBB (Event Based Branches) on 64-bit book3s. See the
included documentation for more details.

EBBs are a feature which allows the hardware to branch directly to a
specified user space address when a PMU event overflows. This can be
used by programs for self-monitoring with no kernel involvement in the
inner loop.

Most of the logic is in the generic book3s code, primarily to avoid a
proliferation of PMU callbacks.
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>