powerpc: Exception hooks for context tracking subsystem

powerpc: Exception hooks for context tracking subsystem

This is the exception hooks for context tracking subsystem, including
data access, program check, single step, instruction breakpoint, machine check,
alignment, fp unavailable, altivec assist, unknown exception, whose handlers
might use RCU.

This patch corresponds to
[PATCH] x86: Exception hooks for userspace RCU extended QS
  commit e2792af6be4f6bd8aee7ec824dfdf5db7edc6e49

But after the exception handling moved to generic code, and some changes in
following two commits:
d1dbbc25dab29b49db585a93e36e78853824a2e6
  context_tracking: Move exception handling to generic code
91b7c74189cc5eb8a0648587a17b970ca7c16673
  context_tracking: Restore correct previous context state on exception exit

it is able for exception hooks to use the generic code above instead of a
redundant arch implementation.

Signed-off-by: Li Zhong <zhong@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>