target/ppc: introduce get_cpu_vsr{l,h}() and set_cpu_vsr{l,h}() helpers for VSR register access

These helpers allow us to move VSR register values to/from the specified TCGv_i64
argument.

To prevent VSX helpers accessing the cpu_vsr array directly, add extra TCG
temporaries as required.
Signed-off-by: NMark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: NRichard Henderson <richard.henderson@linaro.org>
Acked-by: NDavid Gibson <david@gibson.dropbear.id.au>
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>