selftests/powerpc: Move get_auxv_entry() to harness.c

Move get_auxv_entry() from pmu/lib.c up to harness.c in order to make
it available to other tests.

Signed-off-by: Sam Bobroff <sam.bobroff@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>