powerpc: sstep: Add tests for compute type instructions

This enhances the current selftest framework for validating
the in-kernel instruction emulation infrastructure by adding
support for compute type instructions i.e. integer ALU-based
instructions. Originally, this framework was limited to only
testing load and store instructions.

While most of the GPRs can be validated, support for SPRs is
limited to LR, CR and XER for now.

When writing the test cases, one must ensure that the Stack
Pointer (GPR1) or the Thread Pointer (GPR13) are not touched
by any means as these are vital non-volatile registers.

Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>
[mpe: Use patch_site for the code patching]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>