[POWERPC] Add SPE registers to core dumps

This makes the SPE register data appear in ELF core dumps, using the
new n_type value NT_PPC_SPE (0x101).  This new note type is not used
by any consumers of core files yet, but support can be added.  I don't
even have any hardware with SPE capabilities, so I've never seen such
a note.  But this demonstrates how simple it is to export register
information in core dumps when the user_regset style is used for the
low-level code.

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