selftests/powerpc: Force ptrace tests to build -fno-pie

Currently these tests won't build with a `--enable-default-pie`
compiler as they require r30 to be clobbered. This gives
an error:
  ptrace-tm-spd-gpr.c:41:2: error: PIC register clobbered by 'r30' in 'asm'

This forces these tests to be built no-pie.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>