selftests/powerpc: Add more utility macros

Adds _MSG assertion variants to provide more context behind why a
failure occurred. Also include unistd.h for _exit() and stdio.h for
fprintf(), and move ARRAY_SIZE macro to utils.h.

The _MSG variants and ARRAY_SIZE will be used by the following
DEXCR selftests.

Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
Reviewed-by: Russell Currey <ruscur@russell.cc>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230616034846.311705-10-bgray@linux.ibm.com