target/ppc: introduce get_fpr() and set_fpr() helpers for FP register access

target/ppc: introduce get_fpr() and set_fpr() helpers for FP register access

These helpers allow us to move FP register values to/from the specified TCGv_i64
argument in the VSR helpers to be introduced shortly.

To prevent FP helpers accessing the cpu_fpr array directly, add extra TCG
temporaries as required.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>