selftests/powerpc/ptrace: Declare test temporary variables as volatile

While the target is volatile, the temporary variables used to access the
target cast away the volatile. This is undefined behaviour, and a
compiler may optimise away/reorder these accesses, breaking the test.

This was observed with GCC 13.1.1, but it can be difficult to reproduce
because of the dependency on compiler behaviour.

Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230725005841.28854-5-bgray@linux.ibm.com