selftests/powerpc: Update strlen() test to test the new assembly function for PPC32

This patch adds a test for testing the new assembly strlen() for PPC32

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
[mpe: Fix 64-bit build]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>