powerpc/ptrace: split out VSX related functions.

Move CONFIG_VSX functions out of ptrace.c, into
ptrace-vsx.c and ptrace-novsx.c

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/dc8e20c8c95b7e83add0c6dd48f9470628896c5c.1582848567.git.christophe.leroy@c-s.fr