[POWERPC] Switch to using user_regset-based core dumps

This switches powerpc to using the user_regset-based code for ELF core
dumps.  The core dumps come out exactly the same either way, except that
the NT_PPC_VMX note is now omitted for any thread that never touched its
Altivec registers (thread_struct.vr_used).

Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>