powerpc: emulate_step() tests for load/store instructions

Add new selftest that test emulate_step for Normal, Floating Point,
Vector and Vector Scalar - load/store instructions. Test should run
at boot time if CONFIG_KPROBES_SANITY_TEST and CONFIG_PPC64 is set.

Sample log:

  emulate_step_test: ld             : PASS
  emulate_step_test: lwz            : PASS
  emulate_step_test: lwzx           : PASS
  emulate_step_test: std            : PASS
  emulate_step_test: ldarx / stdcx. : PASS
  emulate_step_test: lfsx           : PASS
  emulate_step_test: stfsx          : PASS
  emulate_step_test: lfdx           : PASS
  emulate_step_test: stfdx          : PASS
  emulate_step_test: lvx            : PASS
  emulate_step_test: stvx           : PASS
  emulate_step_test: lxvd2x         : PASS
  emulate_step_test: stxvd2x        : PASS

Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.vnet.ibm.com>
[mpe: Drop start/complete lines, make it all __init]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>