selftests/powerpc/ptrace: Build the ptrace-gpr test as 32-bit when possible

The ptrace-gpr test can now be built 32-bit, so do that if that's the
compiler default rather than forcing a 64-bit build.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220627140239.2464900-9-mpe@ellerman.id.au